This CL migrates the CPP builtin to CSA with fast paths for strings
that can be unpacked to direct one-byte strings. Short strings are
handled directly in CSA, others need to call into C for conversion.

Microbenchmarks for "abcd".toLowerCase() show speedups of 2.5x.

BUG=v8:6353,v8:6344

Review-Url: https://codereview.chromium.org/2859203002
Cr-Commit-Position: refs/heads/master@{#45141}