string_bytes: implement new v8 String API

Use the new String::NewFrom* methods to write out strings. Performance
improvements are subtle, except in the case of ASCII. There we gain ~60%
in performance.

Also added node_isolate to HandleScopes.
string_bytes: use external strings when large

In cases of large one byte strings, and all cases of two byte strings,
it's faster to use an External(Ascii)StringResource. The classes have
been kept hidden from devs, but might be useful to expose the ::New and
::NewFromCopy methods in some way.
string_bytes: binary use one byte strings

Noticed that NewFromOneByte had the same result as the previous
implementation.