This is needed for bug 1500926. It takes the approach of taking a JSFlatString
and using AutoAssertCannotGC to read the memory directly from the JS heap.
This lets us avoid re-encoding strings when performing lookups, which can be
advantageous.
Only ASCII strings are supported by this handler, and wide strings are hashed
as though they contain only values under 0x7f. This is OK as invalid keys to
perfecthash may return any hashtable entry.
Differential Revision: https://phabricator.services.mozilla.com/D9405
Sylvestre Ledru
Nika Layzell