GeneratorFunction() makes generator instances

The current specification has GeneratorFunction() be like Function(),
except that it makes generator instances.  This commit implements that
behavior.  It also fills in a piece of the implementation where
otherwise calling GeneratorFunction or GeneratorFunctionPrototype would
cause an abort because they have no code.

R=mstarzinger@chromium.org
TEST=mjsunit/harmony/generators-iteration
TEST=mjsunit/harmony/generators-runtime
BUG=v8:2355,v8:2680

Review URL: https://codereview.chromium.org/15218004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15084 ce2b1a6d-e550-0410-aec6-3dcde31c8c00