- Introduce a class JSReceiver, that is a common superclass of JSObject and
  JSProxy. Use JSReceiver where appropriate (probably lots of places that we
  still have to migrate, but we will find those later with proxy test suite).

- Move appropriate methods to JSReceiver class (SetProperty,
  GetPropertyAttribute, Get/SetPrototype, Lookup, and so on).

- Introduce new JSFunctionProxy subclass of JSProxy. Currently only a stub.

- Overhaul enum InstanceType:
  * Introduce FIRST/LAST_SPEC_OBJECT_TYPE that ranges over all types that
    represent JS objects, and use that consistently to check language types.
  * Rename FIRST/LAST_JS_OBJECT_TYPE and FIRST/LAST_FUNCTION_CLASS_TYPE
    to FIRST/LAST_[NON]CALLABLE_SPEC_OBJECT_TYPE for clarity.
  * Eliminate the overlap over JS_REGEXP_TYPE.
  * Also replace FIRST_JS_OBJECT with FIRST_JS_RECEIVER, but only use it where
    we exclusively talk about the internal representation type.
  * Insert JS_PROXY and JS_FUNCTION_PROXY in the appropriate places.

- Fix all checks concerning classification, especially for functions, to
  use the CALLABLE_SPEC_OBJECT range (that includes funciton proxies).

- Handle proxies in SetProperty (that was the easiest part :) ).

- A few simple test cases.

R=kmillikin@chromium.org

Review URL: http://codereview.chromium.org/6992072

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00