Rename executable_semantics to explorer (#1188)

Change generated with:

```
#!/usr/bin/bash -eux

# Helper script for renaming pending work.
# Run from the repo root.

# Rename executable_semantics in code.
sed -i 's/executable_semantics/explorer/g' \
  $(git grep -l 'executable_semantics' . | grep -v proposals)
sed -i 's/executable semantics/explorer/g' \
  $(git grep -l 'executable semantics' . | grep -v proposals)
sed -i 's/Executable semantics/Explorer/g' \
  $(git grep -l 'Executable semantics' . |  grep -v proposals)
sed -i 's/Executable Semantics/Explorer/g' \
  $(git grep -l 'Executable Semantics' . |  grep -v proposals)
sed -i 's/EXECUTABLE_SEMANTICS/EXPLORER/g' \
  $(git grep -l 'EXECUTABLE_SEMANTICS' . | grep -v proposals)
sed -i 's/ExecutableSemantics/Explorer/g' \
  $(git grep -l 'ExecutableSemantics' . | grep -v proposals)
sed -i 's/executable-semantics/explorer/g' \
  $(git grep -l 'executable-semantics' . | grep -v proposals)

# This is only needed for the initial move.
mv executable_semantics explorer
mv explorer/fuzzing/executable_semantics_fuzzer.cpp explorer/fuzzing/explorer_fuzzer.cpp
```

Verified with `bazel test ...`

explorer/ast/expression_test.cpp

@@ -0,0 +1,138 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "explorer/ast/expression.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <string>
+
+#include "explorer/ast/paren_contents.h"
+#include "explorer/common/arena.h"
+#include "llvm/Support/Casting.h"
+
+namespace Carbon::Testing {
+namespace {
+
+using llvm::cast;
+using testing::ElementsAre;
+using testing::IsEmpty;
+
+// Matches any `IntLiteral`.
+MATCHER(IntField, "") { return arg->kind() == ExpressionKind::IntLiteral; }
+
+static auto FakeSourceLoc(int line_num) -> SourceLocation {
+  return SourceLocation("<test>", line_num);
+}
+
+class ExpressionTest : public ::testing::Test {
+ protected:
+  Arena arena;
+};
+
+TEST_F(ExpressionTest, EmptyAsExpression) {
+  ParenContents<Expression> contents = {.elements = {},
+                                        .has_trailing_comma = false};
+  Nonnull<const Expression*> expression =
+      ExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(expression->source_loc(), FakeSourceLoc(1));
+  ASSERT_EQ(expression->kind(), ExpressionKind::TupleLiteral);
+  EXPECT_THAT(cast<TupleLiteral>(*expression).fields(), IsEmpty());
+}
+
+TEST_F(ExpressionTest, EmptyAsTuple) {
+  ParenContents<Expression> contents = {.elements = {},
+                                        .has_trailing_comma = false};
+  Nonnull<const Expression*> tuple =
+      TupleExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(tuple->source_loc(), FakeSourceLoc(1));
+  ASSERT_EQ(tuple->kind(), ExpressionKind::TupleLiteral);
+  EXPECT_THAT(cast<TupleLiteral>(*tuple).fields(), IsEmpty());
+}
+
+TEST_F(ExpressionTest, UnaryNoCommaAsExpression) {
+  // Equivalent to a code fragment like
+  // ```
+  // (
+  //   42
+  // )
+  // ```
+  ParenContents<Expression> contents = {
+      .elements = {arena.New<IntLiteral>(FakeSourceLoc(2), 42)},
+      .has_trailing_comma = false};
+
+  Nonnull<const Expression*> expression =
+      ExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(expression->source_loc(), FakeSourceLoc(2));
+  ASSERT_EQ(expression->kind(), ExpressionKind::IntLiteral);
+}
+
+TEST_F(ExpressionTest, UnaryNoCommaAsTuple) {
+  ParenContents<Expression> contents = {
+      .elements = {arena.New<IntLiteral>(FakeSourceLoc(2), 42)},
+      .has_trailing_comma = false};
+
+  Nonnull<const Expression*> tuple =
+      TupleExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(tuple->source_loc(), FakeSourceLoc(1));
+  ASSERT_EQ(tuple->kind(), ExpressionKind::TupleLiteral);
+  EXPECT_THAT(cast<TupleLiteral>(*tuple).fields(), ElementsAre(IntField()));
+}
+
+TEST_F(ExpressionTest, UnaryWithCommaAsExpression) {
+  ParenContents<Expression> contents = {
+      .elements = {arena.New<IntLiteral>(FakeSourceLoc(2), 42)},
+      .has_trailing_comma = true};
+
+  Nonnull<const Expression*> expression =
+      ExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(expression->source_loc(), FakeSourceLoc(1));
+  ASSERT_EQ(expression->kind(), ExpressionKind::TupleLiteral);
+  EXPECT_THAT(cast<TupleLiteral>(*expression).fields(),
+              ElementsAre(IntField()));
+}
+
+TEST_F(ExpressionTest, UnaryWithCommaAsTuple) {
+  ParenContents<Expression> contents = {
+      .elements = {arena.New<IntLiteral>(FakeSourceLoc(2), 42)},
+      .has_trailing_comma = true};
+
+  Nonnull<const Expression*> tuple =
+      TupleExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(tuple->source_loc(), FakeSourceLoc(1));
+  ASSERT_EQ(tuple->kind(), ExpressionKind::TupleLiteral);
+  EXPECT_THAT(cast<TupleLiteral>(*tuple).fields(), ElementsAre(IntField()));
+}
+
+TEST_F(ExpressionTest, BinaryAsExpression) {
+  ParenContents<Expression> contents = {
+      .elements = {arena.New<IntLiteral>(FakeSourceLoc(2), 42),
+                   arena.New<IntLiteral>(FakeSourceLoc(3), 42)},
+      .has_trailing_comma = true};
+
+  Nonnull<const Expression*> expression =
+      ExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(expression->source_loc(), FakeSourceLoc(1));
+  ASSERT_EQ(expression->kind(), ExpressionKind::TupleLiteral);
+  EXPECT_THAT(cast<TupleLiteral>(*expression).fields(),
+              ElementsAre(IntField(), IntField()));
+}
+
+TEST_F(ExpressionTest, BinaryAsTuple) {
+  ParenContents<Expression> contents = {
+      .elements = {arena.New<IntLiteral>(FakeSourceLoc(2), 42),
+                   arena.New<IntLiteral>(FakeSourceLoc(3), 42)},
+      .has_trailing_comma = true};
+
+  Nonnull<const Expression*> tuple =
+      TupleExpressionFromParenContents(&arena, FakeSourceLoc(1), contents);
+  EXPECT_EQ(tuple->source_loc(), FakeSourceLoc(1));
+  ASSERT_EQ(tuple->kind(), ExpressionKind::TupleLiteral);
+  EXPECT_THAT(cast<TupleLiteral>(*tuple).fields(),
+              ElementsAre(IntField(), IntField()));
+}
+
+}  // namespace
+}  // namespace Carbon::Testing