Previously there has been no reason to context-allocate the receiver, so
access to the receiver always goes through the stack.  This was failing
with generators, which assumed that forcing context allocation would
relieve the need of storing anything but the context and the function on
the stack.

This CL adds a slot in generator objects to capture the receiver, and
restores it when resuming a generator.

BUG=v8:2355
TEST=mjsunit/harmony/generators-iteration

Review URL: https://codereview.chromium.org/14158006

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

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14434 ce2b1a6d-e550-0410-aec6-3dcde31c8c00