Generator objects can suspend

* src/ast.h:
* src/parser.cc: Differentiate between the different kinds of yields, in
  anticipation of boxing return values.  Parse `return' into `yield' in
  a generator.

* src/runtime.h:
* src/runtime.cc (Runtime_SuspendJSGeneratorObject): New horrible
  runtime function: saves continuation, context, and operands into the
  generator object.

* src/arm/full-codegen-arm.cc (VisitYield):
* src/ia32/full-codegen-ia32.cc (VisitYield):
* src/x64/full-codegen-x64.cc (VisitYield): Arrange to call
  SuspendJSGeneratorObject.  If the call returns the hole, we suspend.
  Otherwise we resume.

BUG=v8:2355
TEST=These codepaths are tested when the generator is first invoked, and so
are covered by mjsunit/harmony/generators-objects.js.

Review URL: https://codereview.chromium.org/13704010

Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14353 ce2b1a6d-e550-0410-aec6-3dcde31c8c00