Code review for simplehash.h.

Code review for simplehash.h.

Fix reference to non-existent file in comment.

Add SH_ prefix to the EMPTY and IN_USE tokens, to reduce likelihood of
collisions with unrelated macros.

Add include guards around the function definitions that are not
"parameterized", so the header can be used again in the same translation
unit.

Undefine SH_EQUAL macro where other "parameter" macros are undefined, for
the same reason.

Author: Thomas Munro
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAEepm%3D1LdXZ3mMTM8tHt_b%3DK1kREit%3Dp8sikesak%3DkzHHM07Nw%40mail.gmail.com