Add experimental support for external two-byte symbols....

Add experimental support for external two-byte symbols.

This might be a big win in a browser setting where a lot of string
conversions can be avoided.  On the other hand it adds extra pressure
on the global handle system.

Committed: http://code.google.com/p/v8/source/detail?r=772

[Kasper Lund, 2008-11-17 15:06:58]

LGTM, but you should document that you're passing UTF8 data to the callback
function somewhere.