This is a preparatory step for implementing generics. Along the way, clean up
and encapsulate a bunch of code, including:

* Fully encapsulate Scope by adding the new class ScopeChain that provide an
  abstraction for creating and activating scopes.
* Untangle Modules and Scopes.
* Unify scope activation so that it is always associated with an AST node
  and triggered by a RAII helper class.
* Unify (somewhat) how builtins and macros are created, fixing a few
  inconsistencies with when and how parameters and their types are declared.
* Create a new Declarations class that brokers between the visitor classes and
  the ScopeChain. This moves handling of declaration-related errors out of the
  visitors but also makes it possible to do so without polluting Scope and
  ScopeChain with details about resolving SourcePositions in error cases.

Change-Id: I180017d4cf39ccf5ef1d20b84f53284c252f8d87
Reviewed-on: https://chromium-review.googlesource.com/1038504
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52947}