Add parser support for generators.

This patchset begins by adding support for "yield", which is unlike other tokens
in JS. In a generator, whether strict or classic, it is a syntactic keyword.
In classic mode it is an identifier. In strict mode it is reserved.

This patch adds YIELD as a token to the scanner, and adapts the preparser and
parser appropriately. It also parses "function*", indicating that a function is
actually a generator, for both eagerly and lazily parsed functions.

Currently "yield" just compiles as "return".

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

Review URL: https://codereview.chromium.org/12646003
Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14116 ce2b1a6d-e550-0410-aec6-3dcde31c8c00