Disallow sizes with increasing unit size

Things like 3B4T, 4B50B, 400 100 (meaning 4*1024**4+3, 54, and 500,
respectively) are now disallowed. It is necessary to say 4T3B, 54B,
500 instead. I think this was confusing and error prone.

As a special form, 400B 100 is allowed, i.e. "B" suffix is treated
as different from "", although they mean the same thing.