Generators return boxed values

Generators now box their return values in object literals of the form

  { value: VAL, done: DONE }

where DONE is false for yield expressions, and true for return
statements.

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

Review URL: https://codereview.chromium.org/13870007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14563 ce2b1a6d-e550-0410-aec6-3dcde31c8c00