This cleans up string-util.h a little. Instead of distinguishing
BinaryStringBuffer from StringBufferImpl (which has a legacy
::adopt() interface), provide two functions for making StringBuffer
from either an 8 bit (std::vector<uint8_t>) or a 16 bit (String16)
source, and keep the classes in the .cc file only. Since it's easy
enough, this adds a class also for the empty string buffer as there's
no need to keep an empty vector / String16 this way.

No public API changes here.

Change-Id: Idb25fe24ea94f27f8001d552cede089e04eacd32
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2016015Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65964}