Compute MAKE-INTEGER's magic constants for any n-fixnum-bits.

Also rename token-buf's OUCH-PTR to FILL-PTR.
LOOP: produce warnings for empty types.

Jan Moringen reported that the test
(typep (loop with a of-type extended-char return a) 'extended-char)
failed on #-sb-unicode.
It returned 0.0 and didn't produce a warning during compilation, since
extended-char is an empty type on #-sb-unicode.
Catch empty types and don't produce default values for them.
Change some more uses of INCH-READ-BUFFER to TOKEN-BUF-GETCHAR.

Also delete READ-UNWIND-READ-BUFFER which seemed to confuse
rewinding with unwinding - I would think that an unwound tape
couldn't be read again but a rewound one could.