Implement SQL99 CREATE CAST and DROP CAST statements.

 Also implement alternative forms to expose the PostgreSQL CREATE FUNCTION
 features.
Implement syntax for READ ONLY and READ WRITE clauses in SET TRANSACTION.
 READ WRITE is already implemented (of course).
Implement syntax for "LIKE table" clause in CREATE TABLE. Should be fairly
 easy to complete since it resembles SELECT INTO.
Implement MATCH SIMPLE clause for foreign key definitions. This is explicit
 SQL99 syntax for the default behavior, so we now support it :)
Start implementation of shorthand for national character literals in
 scanner. For now, just swallow the leading "N", but sometime soon let's
 figure out how to pass leading type info from the scanner to the parser.
 We should use the same technique for binary and hex bit string literals,
 though it might be unusual to have two apparently independent literal
 types fold into the same storage type.