Unified INDEX_CHARP and generic_extract (and thus index_shared_string)

In non-debug mode indexing a shared string is now the same thing as
doing the old INDEX_CHARP, and thus actually noticeably faster.

However, INDEX_CHARP will be slower in debug mode since it will check
if the size_shift is >2. That seems rather harmless, though, since
debug mode is seldom used when performance matters.