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forbid use of Rng::gen_range

This function panics if the range is empty. Prompted by #918 (closed) I did a git grep and there were call sites where it wasn't straightforward to convince myself that everything is fine.

I think we should:

  • Make an extension trait to Rng which provides gen_range_checked which returns Option
  • Change every call site to use that; where panicking on empty ranges is warranted, that should be done explicitly at the call site with expect
  • Somehow arrange to forbid use of gen_range. I'm not sure if clippy can do this for us, but if not then currently this rune seems to work:
git-grep -P '\bgen_range\b(?!`)'

@nickm, @gabi-250 ?