Move hash_any prototype from access/hash.h to utils/hashutils.h

... as well as its implementation from backend/access/hash/hashfunc.c to
backend/utils/hash/hashfn.c.

access/hash is the place for the hash index AM, not really appropriate
for generic facilities, which is what hash_any is; having things the old
way meant that anything using hash_any had to include the AM's include
file, pointlessly polluting its namespace with unrelated, unnecessary
cruft.

Also move the HTEqual strategy number to access/stratnum.h from
access/hash.h.

To avoid breaking third-party extension code, add an #include
"utils/hashutils.h" to access/hash.h.  (An easily removed line by
committers who enjoy their asbestos suits to protect them from angry
extension authors.)

Discussion: https://postgr.es/m/201901251935.ser5e4h6djt2@alvherre.pgsql

src/include/access/stratnum.h

@@ -34,6 +34,13 @@ typedef uint16 StrategyNumber;
 
 #define BTMaxStrategyNumber				5
 
+/*
+ *	Strategy numbers for hash indexes. There's only one valid strategy for
+ *	hashing: equality.
+ */
+#define HTEqualStrategyNumber			1
+
+#define HTMaxStrategyNumber				1
 
 /*
  * Strategy numbers common to (some) GiST, SP-GiST and BRIN opclasses.