Use FastAsciiConvert (as used by Unibrow) for i18n-aware
case conversion with --icu_case_mapping.

Move FastAsciiConvert to src/string-case.cc so that it can be used
by both runtime-{string,i18n}.

Add more tests.

BUG=v8:4477,v8:4476
TEST=intl/general/case*

Review-Url: https://codereview.chromium.org/2533983006
Cr-Commit-Position: refs/heads/master@{#41821}