- Add MICROPY_GC_HOOK_LOOP to gc_info() and gc_alloc(). Both of these
  can be long running (many milliseconds) which is too long to be
  blocking in some applications.
- Pass loop variable to MICROPY_GC_HOOK_LOOP(i) macro so that
  implementers can use it, e.g. to improve performance by only calling
  a function every X number of iterations.
- Drop outer call to MICROPY_GC_HOOK_LOOP in gc_mark_subtree().