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/static_scope.cpp

@@ -0,0 +1,57 @@
+// 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/static_scope.h"
+
+#include "explorer/common/error_builders.h"
+#include "llvm/Support/Error.h"
+
+namespace Carbon {
+
+auto StaticScope::Add(std::string name, ValueNodeView entity)
+    -> ErrorOr<Success> {
+  auto [it, success] = declared_names_.insert({name, entity});
+  if (!success && it->second != entity) {
+    return CompilationError(entity.base().source_loc())
+           << "Duplicate name `" << name << "` also found at "
+           << it->second.base().source_loc();
+  }
+  return Success();
+}
+
+auto StaticScope::Resolve(const std::string& name,
+                          SourceLocation source_loc) const
+    -> ErrorOr<ValueNodeView> {
+  ASSIGN_OR_RETURN(std::optional<ValueNodeView> result,
+                   TryResolve(name, source_loc));
+  if (!result) {
+    return CompilationError(source_loc) << "could not resolve '" << name << "'";
+  }
+  return *result;
+}
+
+auto StaticScope::TryResolve(const std::string& name,
+                             SourceLocation source_loc) const
+    -> ErrorOr<std::optional<ValueNodeView>> {
+  auto it = declared_names_.find(name);
+  if (it != declared_names_.end()) {
+    return std::make_optional(it->second);
+  }
+  std::optional<ValueNodeView> result;
+  for (Nonnull<const StaticScope*> parent : parent_scopes_) {
+    ASSIGN_OR_RETURN(std::optional<ValueNodeView> parent_result,
+                     parent->TryResolve(name, source_loc));
+    if (parent_result.has_value() && result.has_value() &&
+        *parent_result != *result) {
+      return CompilationError(source_loc)
+             << "'" << name << "' is ambiguous between "
+             << result->base().source_loc() << " and "
+             << parent_result->base().source_loc();
+    }
+    result = parent_result;
+  }
+  return result;
+}
+
+}  // namespace Carbon