Move memory context callback declarations into palloc.h.

Initial experience with this feature suggests that instances of
MemoryContextCallback are likely to propagate into some widely-used headers
over time.  As things stood, that would result in pulling memutils.h or
at least memnodes.h into common headers, which does not seem desirable.
Instead, let's decide that this feature is part of the "ordinary palloc
user" API rather than the "specialized context management" API, and as
such should be declared in palloc.h not memutils.h.