Fix bugs in %OBJECT-REF-/MEMREF-SIGNED-BYTE-64.
Fix bugs in %OBJECT-REF-/MEMREF-SIGNED-BYTE-64.

%OBJECT-REF-SIGNED-BYTE-64 was not boxing 64-bit values properly
(SETF %OBJECT-REF-/MEMREF-SIGNED-BYTE-64) were not checking types
properly, leading to type errors when storing bignums.

varjagg/Mezzano,froggey/Mezzano,froggey/Mezzano,rvedam/Mezzano,rvedam/Mezzano,varjagg/Mezzano