fix CT regex https://github.com/nitely/nim-regex/issues/4

[timotheecour]

/cc @nitely please don't forget to create new git tag for both nim-regex and nim-unicodedb after merging!

• fixes CT regex Support matching at compile-time nim-regex#4
  this is an ugly hack but it works; i was hoping something like increase dfa instruction limit nim-lang/Nim#11579 could've been used instead but i didn't see how to make it work as the assumption -0x7fff < diff and diff < 0x7fff seems rather deeply rooted in vmgen

note

CI fails for 0.18.0 (works for 0.19); although i could try to make it work for 0.18, how about dropping support for 0.18 (quite old now) and adding a CI for 0.20 instead?
https://travis-ci.org/nitely/nim-unicodedb/jobs/553541005

/usr/src/app/src/unicodedb/types.nim(38, 28) Error: type expected

[nitely]

This is awful and clever and I like it. Make tests green and I'll merge it.

[timotheecour]

green!

[nitely]

Thanks!

PS. There is no need to release a new regex version, I think.

[timotheecour]

@nitely
nitely/nim-regex#4 is likely root-caused by nim-lang/Nim#12727 ; and nim-lang/Nim#12777 might make this change obsolete; might be worth making this hack conditional:
when (NimMajor, NimMinor) < (x, y): ...

[nitely]

@timotheecour ah, yes. I'll keep an eye on that PR, and test later. Thanks!

[timotheecour]

that PR nim-lang/Nim#12777 just got merged