GiST header cleanup

This patch moves GiST implementation details from gist.h into a new
header file, gist_private.h. gist.h should only contain APIs that are
exposed to clients writing GiST extensions -- where possible we should
avoid backward-incompatible changes to those APIs, so it makes sense to
keep that API in a separate file.

Other related changes:

- pruned down the list of unnecessary includes in gist.h; as a result I
had to add a missing #include <float.h> to contrib/cube/cube.c

- remove isAttByVal(), which is no longer used

- remove declaration of _gistdump(), which is never defined

I noticed that GISTNStrategies is defined, but never used; instead there
is a literal "100" in include/catalog/pg_am.h. Does anyone see a reason
to keep GISTNStrategies around? Alternatively, should pg_am.h include
gist.h and reference GISTNStrategies instead of using "100"?

All of contrib/ continues to compile without warnings with this patch; I
haven't tried externally maintained GiST extensions, but they may need a
bit of #include tweaking.

Barring any objections I'll apply this later today or tomorrow.

-Neil