indxpath.c refactoring

This simple refactoring patch moves all of the code associated with
Partial Index planning into a single file. This isolates it from other
code changes that may take place over the next few months, as well as
setting the scene for a number of changes that will hopefully take place
with that code.

This should allow work to continue on this undisturbed; more work will
happen within the 8.1 window, so please approve this patch to cvstip
now.

Changes likely on this code are
- redesigning the low level routines to allow them to perform their
stuff for both Partial Index and Constraint-based elimination (aka
Partitioning...)
- allowing equivalence class re-write
- allow handling for r-tree operators
- maybe GIST also??

Better to do this change now, than try to do it when code gets more
complex.

- Passes make check on current cvstip.

- No functional code changes of any kind, just moving between modules.

Changes:
- code removal from indxpath.c
- code addition to new module predtest.c
- no changes required to header files
- update Makefile

Best Regards, Simon Riggs