Tune bytecode quitting

Tune bytecode quitting

* src/bytecode.c (BYTE_CODE_QUIT): Check for GC, too.  Do the
check only once every 256 times.  This should be good enough, and
improves performance significantly on x86-64 as branch-prediction
typically assumes checking will not be done so the instruction
pipeline stays fuller.
(exec_byte_code): Set up the quit counter.  Don’t call maybe_gc
directly, as BYTE_CODE_QUIT does that now.