Bug 666399 - New Harmony syntax for generators. r=Waldo

Add a GeneratorKind enumeration, and use it in the parser and runtime to
indicate whether a function is a non-generator, a legacy generator, or a
star generator.

Always parse "yield" as TOK_YIELD, regardless of the version.  Add
TokenStream::currentName() to retrieve the current token's name, which
works for TOK_NAME or TOK_YIELD.  The parser should check the validity
of "yield" as a name, if needed, using checkYieldNameValidity().

Parse "function*" as the StarGenerator GeneratorKind, and allow star
generators to be lazily parsed.

Separate parsing of return statements from yield expressions.

js/src/jsatom.cpp

@@ -104,7 +104,6 @@ const char js_typeof_str[]          = "typeof";
 const char js_void_str[]            = "void";
 const char js_while_str[]           = "while";
 const char js_with_str[]            = "with";
-const char js_yield_str[]           = "yield";
 
 /*
  * For a browser build from 2007-08-09 after the browser starts up there are