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/action.cpp

@@ -2,6 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include "executable_semantics/interpreter/action.h"
+
 #include <iostream>
 #include <iterator>
 #include <map>
@@ -11,9 +13,7 @@
 
 #include "executable_semantics/ast/expression.h"
 #include "executable_semantics/ast/function_definition.h"
-#include "executable_semantics/interpreter/interpreter.h"
 #include "executable_semantics/interpreter/stack.h"
-#include "executable_semantics/interpreter/typecheck.h"
 
 namespace Carbon {