Port bufpage.h functions

[rebasedming]

Ports functions from bufpage.h that are used to interact with block storage

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Just the one change and I think this will be good to merge.

Thanks, @rebasedming

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For completeness, this should do what the BufferIsValid() macro does too. Can you add another top-level function for that and then assert!(BufferIsValid(buffer)) here, too?

/*
 * BufferIsValid
 *		True iff the given buffer number is valid (either as a shared
 *		or local buffer).
 *
 * Note: For a long time this was defined the same as BufferIsPinned,
 * that is it would say False if you didn't hold a pin on the buffer.
 * I believe this was bogus and served only to mask logic errors.
 * Code should always know whether it has a buffer reference,
 * independently of the pin state.
 *
 * Note: For a further long time this was not quite the inverse of the
 * BufferIsInvalid() macro, in that it also did sanity checks to verify
 * that the buffer number was in range.  Most likely, this macro was
 * originally intended only to be used in assertions, but its use has
 * since expanded quite a bit, and the overhead of making those checks
 * even in non-assert-enabled builds can be significant.  Thus, we've
 * now demoted the range checks to assertions within the macro itself.
 */
static inline bool
BufferIsValid(Buffer bufnum)
{
	Assert(bufnum <= NBuffers);
	Assert(bufnum >= -NLocBuffer);

	return bufnum != InvalidBuffer;
}

I think the rust port would basically be:

/// <comments from above, please>
pub unsafe fn BufferIsValid(bufnum: pg_sys::Buffer) -> bool {
   assert!(bufnum <= pg_sys::NBuffers);
   assert!(bufnum >= -pg_sys::NLocBuffer);
   bufnum != pg_sys::InvalidBuffer
}

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oops, requesting changes

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I'll figure out what's wrong with CI -- appears to be unrelated to this PR.

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Thank you @rebasedming. Merging!