Bug in beluga-mode holes

[sarahzrf]

beluga-mode's holes feature does not work after the first load as of https://github.com/Beluga-lang/Beluga/commit/1d39095c99dc255d972a339d447dc04286d8c13f because the elisp assumes that holes are numbered starting from 0 but the ocaml uses a single counter to assign numbers to holes across multiple file loads.

Here's a sample patch that fixes the problem, albeit in a possibly non-ideal way: https://gist.github.com/ee727345c639c6d496c6e177f955fe57