Move pg_constraint.h function declarations to new file pg_constraint_fn.h.

Move pg_constraint.h function declarations to new file pg_constraint_fn.h.

A pending patch requires exporting a function returning Bitmapset from
catalog/pg_constraint.c.  As things stand, that would mean including
nodes/bitmapset.h in pg_constraint.h, which might be hazardous for the
client-side includability of that header.  It's not entirely clear whether
any client-side code needs to include pg_constraint.h, but it seems prudent
to assume that there is some such code somewhere.  Therefore, split off the
function definitions into a new file pg_constraint_fn.h, similarly to what
we've done for some other catalog header files.