Bring the C and Python compute_hash functions into consistency
Merge pull request #670 from Rosuav/stringhash

Bring the C and Python compute_hash functions into consistency
Add PEP 393-flags to strings and stub usage.

The test suite all passes, but nothing has actually been changed.
Update comments - now aiming for UTF-8 rather than PEP 393 strings
Beginnings of UTF-8 support - construct strings from that many UTF-8-encoded chars, and subscript bytes the same way
objstr: Record character length separately from byte length

CAUTION: Buggy, may crash stuff - qstr needs equivalent functionality too
HACK - When indexing a qstr, count its charlen. Stupidly inefficient but POC.

All tests pass now, though string creation is still buggy.
Make len(s) return character length (even though creation's still buggy)
Add test of UTF-8 encoded source file resulting in properly formed string
Add character length to qstr