Change collation cache to use simplehash.h.
Speeds up text comparison expressions when using a collation other
than the database default collation. Does not affect larger operations
such as ORDER BY, because the lookup is only done once.
Discussion: https://postgr.es/m/
7bb9f018d20a7b30b9a7f6231efab1b5e50c7720.camel@j-davis.com
Reviewed-by: John Naylor, Andreas Karlsson