Add Nim 1.6.8

[ee7]

Links:

• https://nim-lang.org/blog/2022/09/27/version-168-released.html
• https://github.com/nim-lang/Nim/compare/v1.6.6...v1.6.8
• https://forum.nim-lang.org/t/9494

Previous bump: #21

---

Double-checking that it's the correct nightly release:

$ cd /tmp
$ curl -sSfL -o 1.tar.xz https://nim-lang.org/download/nim-1.6.8-linux_x64.tar.xz
$ curl -sSfL -o 2.tar.xz https://github.com/nim-lang/nightlies/releases/download/2022-09-27-version-1-6-c9f46ca8c9eeca8b5f68591b1abe14b962f80a4c/nim-1.6.8-linux_x64.tar.xz
$ diff -s 1.tar.xz 2.tar.xz
Files 1.tar.xz and 2.tar.xz are identical

[iffy]

Looks good to me! Thanks for making this PR. Once the builds pass I'll merge.

[ee7]

Thanks for the quick release!