Collations with nondeterministic comparison

This adds a flag "deterministic" to collations.  If that is false,
such a collation disables various optimizations that assume that
strings are equal only if they are byte-wise equal.  That then allows
use cases such as case-insensitive or accent-insensitive comparisons
or handling of strings with different Unicode normal forms.

This functionality is only supported with the ICU provider.  At least
glibc doesn't appear to have any locales that work in a
nondeterministic way, so it's not worth supporting this for the libc
provider.

The term "deterministic comparison" in this context is from Unicode
Technical Standard #10
(https://unicode.org/reports/tr10/#Deterministic_Comparison).

This patch makes changes in three areas:

- CREATE COLLATION DDL changes and system catalog changes to support
  this new flag.

- Many executor nodes and auxiliary code are extended to track
  collations.  Previously, this code would just throw away collation
  information, because the eventually-called user-defined functions
  didn't use it since they only cared about equality, which didn't
  need collation information.

- String data type functions that do equality comparisons and hashing
  are changed to take the (non-)deterministic flag into account.  For
  comparison, this just means skipping various shortcuts and tie
  breakers that use byte-wise comparison.  For hashing, we first need
  to convert the input string to a canonical "sort key" using the ICU
  analogue of strxfrm().

Reviewed-by: Daniel Verite <daniel@manitou-mail.org>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://www.postgresql.org/message-id/flat/1ccc668f-4cbc-0bef-af67-450b47cdfee7@2ndquadrant.com

src/include/executor/executor.h

@@ -111,6 +111,7 @@ extern ExprState *execTuplesMatchPrepare(TupleDesc desc,
 					   int numCols,
 					   const AttrNumber *keyColIdx,
 					   const Oid *eqOperators,
+					   const Oid *collations,
 					   PlanState *parent);
 extern void execTuplesHashPrepare(int numCols,
 					  const Oid *eqOperators,
@@ -121,6 +122,7 @@ extern TupleHashTable BuildTupleHashTable(PlanState *parent,
 					int numCols, AttrNumber *keyColIdx,
 					const Oid *eqfuncoids,
 					FmgrInfo *hashfunctions,
+					Oid *collations,
 					long nbuckets, Size additionalsize,
 					MemoryContext tablecxt,
 					MemoryContext tempcxt, bool use_variable_hash_iv);
@@ -129,6 +131,7 @@ extern TupleHashTable BuildTupleHashTableExt(PlanState *parent,
 					int numCols, AttrNumber *keyColIdx,
 					const Oid *eqfuncoids,
 					FmgrInfo *hashfunctions,
+					Oid *collations,
 					long nbuckets, Size additionalsize,
 					MemoryContext metacxt,
 					MemoryContext tablecxt,
@@ -257,6 +260,7 @@ extern ExprState *ExecBuildGroupingEqual(TupleDesc ldesc, TupleDesc rdesc,
 					   int numCols,
 					   const AttrNumber *keyColIdx,
 					   const Oid *eqfunctions,
+					   const Oid *collations,
 					   PlanState *parent);
 extern ProjectionInfo *ExecBuildProjectionInfo(List *targetList,
 						ExprContext *econtext,