Implement IS OF and IS NOT OF type predicate.

Implement IS OF and IS NOT OF type predicate.
 Can now do queries of the form: SELECT value IS OF (integer, float8);
Define macros for handling typmod manipulation for date/time types.
 Should be more robust than all of that brute-force inline code.
Rename macros for masking and typmod manipulation to put TIMESTAMP_
 or INTERVAL_ in front of the macro name, to reduce the possibility
 of name space collisions.
Allow bit string constants without fully-specified length declaration.
Try implementing CREATE TABLE/OF as a mapping to inheritance.
 May be appropriate, or may be replace later with something more exactly
 like one might expect from databases without the feature.