Per-column collation support

This adds collation support for columns and domains, a COLLATE clause
to override it per expression, and B-tree index support.

Peter Eisentraut
reviewed by Pavel Stehule, Itagaki Takahiro, Robert Haas, Noah Misch

src/include/utils/syscache.h

@@ -44,6 +44,8 @@ enum SysCacheIdentifier
 	CASTSOURCETARGET,
 	CLAAMNAMENSP,
 	CLAOID,
+	COLLNAMEENCNSP,
+	COLLOID,
 	CONDEFAULT,
 	CONNAMENSP,
 	CONSTROID,