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/backend/executor/nodeSort.c

@@ -86,6 +86,7 @@ ExecSort(SortState *node)
 											  plannode->numCols,
 											  plannode->sortColIdx,
 											  plannode->sortOperators,
+											  plannode->collations,
 											  plannode->nullsFirst,
 											  work_mem,
 											  node->randomAccess);