* This is mostly just a matter of spreading the PADn lowtags around,
one or two cases of adjusting a constant, and some KLUDGEing around in
the runtime to make everything work for 1 <= n-fixnum-tag-bits <= 3.
* Yes, this means that you can change n-fixnum-tag-bits before
building in order to return to a 61-bit fixnum world. Or you can have
62-bit fixnums if you want.