Don't stringify numeric keys in JS Maps

[bnoordhuis]

First commit is #336, to be rebased away when that PR lands. - edit: rebased

[bnoordhuis]

Suspected flake on linux-amd64 - ruby-3.2 - gnu, rerunning just in case:

free(): corrupted unsorted chunks
........S.......S.......F.S...................................................................

Finished in 3.256550s, 28.8649 runs/s, 47.5964 assertions/s.

  1) Failure:
MiniRacerTest#test_forking [test/mini_racer_test.rb:1042]:
forking test failed

Fork-and-continue is inherently risky so I'm inclined to think that's the cause, not a change in this PR.