ALTER TYPE 3: add facility to identify further no-work cases

Here I add the notion of an "exemptor function", a property of a cast that
determines when calls to the cast would be superfluous. Such calls can be
removed, reduced to RelabelType expressions, or annotated (via a new field in
FuncExpr) with the applicable exemptions. I modify various parse_coerce.c
functions to retrieve, call, and act on these exemptor functions; this includes
GetCoerceExemptions() from the last patch. I did opt to make
find_typmod_coercion_function return COERCION_PATH_RELABELTYPE when no work is
needed, rather than COERCION_PATH_NONE; this makes it consistent with
find_coercion_pathway's use of that enumeration.

To demonstrate the functionality, I add exemptor functions for varchar and xml.
Originally I was only going to start with varchar, but xml tests the other major
code path, and the exemptor function for xml is dead simple.

This helps on conversions like varchar(4)->varchar(8) and text->xml.