Split interpreter into smaller modules (#662)

- Move Heap and Frame/Scope to their own headers.
- Move some functions to more appropriate headers (e.g. CopyValue -> value.h).
- Define a separate Bazel rule for each header/cpp pair.
- Modify PrintValue to not print the frames of a ContinuationValue. This was necessary to break a dependency cycle between PrintValue, PrintFrame, and Action::Print.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/frame.cpp

@@ -0,0 +1,20 @@
+// 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 "executable_semantics/interpreter/frame.h"
+
+#include <ostream>
+
+#include "executable_semantics/interpreter/action.h"
+
+namespace Carbon {
+
+void PrintFrame(Frame* frame, std::ostream& out) {
+  out << frame->name;
+  out << "{";
+  Action::PrintList(frame->todo, out);
+  out << "}";
+}
+
+}  // namespace Carbon