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/parser/parse_node.h

@@ -145,6 +145,7 @@ extern ArrayRef *transformArraySubscripts(ParseState *pstate,
 						 Oid arrayType,
 						 Oid elementType,
 						 int32 arrayTypMod,
+						 Oid arrayColl,
 						 List *indirection,
 						 Node *assignFrom);
 extern Const *make_const(ParseState *pstate, Value *value, int location);