Optimize (mod FIXNUM) type-checks on x86oids.

Instead of two (and (>= x 0) (< x FIXNUM)) comparisons, do one
unsigned.
(mod power-of-two) is further optimized by doing one mask test
determine the range and fixnumness in one go.
Get rid of vm-support-routines indirection.

VM routines were defined using two functions, one calling another
through structure slots. This is unnecessary, removing leads to a
~200KB core size reduction on x86-64.