Rename cryptohashes.c to cryptohashfuncs.c

Rename cryptohashes.c to cryptohashfuncs.c

87ae969 has created two new files called cryptohash{_openssl}.c in
src/common/, whose names overlap with the existing backend file called
cryptohashes.c dedicated to the SQL wrappers for SHA2 and MD5.  This
file is renamed to cryptohashfuncs.c to be more consistent with the
surroundings and reduce the confusion with the new cryptohash interface
of src/common/.

Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/X8hHhaQgbMbW+aGU@paquier.xyz