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/syntax/BUILD

@@ -33,6 +33,7 @@ cc_library(
         "//executable_semantics/ast:declaration",
         "//executable_semantics/ast:expression",
         "//executable_semantics/interpreter",
+        "//executable_semantics/interpreter:typecheck",
     ],
 )