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/syntax/parse.cpp

@@ -0,0 +1,89 @@
+// 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/syntax/parse.h"
+
+#include "common/check.h"
+#include "common/error.h"
+#include "explorer/common/error_builders.h"
+#include "explorer/syntax/lexer.h"
+#include "explorer/syntax/parse_and_lex_context.h"
+#include "explorer/syntax/parser.h"
+#include "llvm/Support/Error.h"
+
+namespace Carbon {
+
+static auto ParseImpl(yyscan_t scanner, Nonnull<Arena*> arena,
+                      std::string_view input_file_name, bool parser_debug)
+    -> ErrorOr<AST> {
+  // Prepare other parser arguments.
+  std::optional<AST> ast = std::nullopt;
+  ParseAndLexContext context(arena->New<std::string>(input_file_name),
+                             parser_debug);
+
+  // Do the parse.
+  auto parser = Parser(arena, scanner, context, &ast);
+  if (parser_debug) {
+    parser.set_debug_level(1);
+  }
+
+  if (auto syntax_error_code = parser(); syntax_error_code != 0) {
+    const std::string error_message = context.error_messages().empty()
+                                          ? "Unknown parser error"
+                                          : context.error_messages()[0];
+    return Error(error_message);
+  }
+
+  // Return parse results.
+  CHECK(ast != std::nullopt)
+      << "parser validated syntax yet didn't produce an AST.";
+  return *ast;
+}
+
+auto Parse(Nonnull<Arena*> arena, std::string_view input_file_name,
+           bool parser_debug) -> ErrorOr<AST> {
+  std::string name_str(input_file_name);
+  FILE* input_file = fopen(name_str.c_str(), "r");
+  if (input_file == nullptr) {
+    return ProgramError(SourceLocation(name_str.c_str(), 0))
+           << "Error opening file: " << std::strerror(errno);
+  }
+
+  // Prepare the lexer.
+  yyscan_t scanner;
+  yylex_init(&scanner);
+  auto buffer = yy_create_buffer(input_file, YY_BUF_SIZE, scanner);
+  yy_switch_to_buffer(buffer, scanner);
+
+  ErrorOr<AST> result =
+      ParseImpl(scanner, arena, input_file_name, parser_debug);
+
+  // Clean up the lexer.
+  yy_delete_buffer(buffer, scanner);
+  yylex_destroy(scanner);
+  fclose(input_file);
+
+  return result;
+}
+
+auto ParseFromString(Nonnull<Arena*> arena, std::string_view input_file_name,
+                     std::string_view file_contents, bool parser_debug)
+    -> ErrorOr<AST> {
+  // Prepare the lexer.
+  yyscan_t scanner;
+  yylex_init(&scanner);
+  auto buffer =
+      yy_scan_bytes(file_contents.data(), file_contents.size(), scanner);
+  yy_switch_to_buffer(buffer, scanner);
+
+  ErrorOr<AST> result =
+      ParseImpl(scanner, arena, input_file_name, parser_debug);
+
+  // Clean up the lexer.
+  yy_delete_buffer(buffer, scanner);
+  yylex_destroy(scanner);
+
+  return result;
+}
+}  // namespace Carbon