Convert uses of hash_string_pointer to fasthash equivalent

Convert uses of hash_string_pointer to fasthash equivalent

Remove duplicate hash_string_pointer() function definitions by creating
a new inline function hash_string() for this purpose.

This has the added advantage of avoiding strlen() calls when doing hash
lookup. It's not clear how many of these are perfomance-sensitive
enough to benefit from that, but the simplification is worth it on
its own.

Reviewed by Jeff Davis

Discussion: https://postgr.es/m/CANWCAZbg_XeSeY0a_PqWmWqeRATvzTzUNYRLeT%2Bbzs%2BYQdC92g%40mail.gmail.com