string_bytes: use external for large strings

When large strings are used they cause v8's GC to spend a lot more time
cleaning up. In these cases it's much faster to use external string
resources.

UTF8 strings do not use external string resources because only one and
two byte external strings are supported.

EXTERN_APEX is the value at which v8's GC overtakes performance.

The following table has the type and buffer size that use to encode the
strings as rough estimates of the percentage of performance gain from
this patch (UTF8 is missing because they cannot be externalized).

encoding  128KB    1MB    5MB
-----------------------------
ASCII       58%   208%   250%
HEX         15%    74%    86%
BASE64      11%    74%    71%
UCS2         2%   225%   398%
BINARY    2234%  1728%  2305%

BINARY is so much faster across the board because of using the new v8
WriteOneByte API.