Shared: JSON Parse Perf

Improve the mechanics for JSON parsing by reusing
text buffers during the parse.

Change-Id: I7fb2cae31e6558a5a0c63fd02e1fc6fec4f9e4b3
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7106
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>