The StringIndexOf fast path used to be very narrow, only allowing
one-byte single-char search strings (and a one-byte subject string).

This changes the CSA fast path to call into our internal SearchString C++
function instead (after attempting to unpack both Strings), and can handle
strings of arbitrary length and encoding. The only remaining runtime call is
when either string needs to be flattened.

BUG=

Review-Url: https://codereview.chromium.org/2814373002
Cr-Commit-Position: refs/heads/master@{#44718}