vm: various minor optimizations speeding up gc0 and dispatch1 benchmarks, as well...

vm: various minor optimizations speeding up gc0 and dispatch1 benchmarks, as well as bootstrap
- make allot_object() inline, move slow case to allot_large_object()
- reduce number of conditional branches in megamorphic cache miss handler
- make unbox_array_size() inline, move slow case to unbox_array_size_slow()
- new memset_cell() function uses memset_pattern4/8() on Mac OS X