add c3

[hwchen]

Add c3 language.

https://github.com/c3lang/c3c
https://c3-lang.org/

Followed the zig implementation.

[hwchen]

On my 10-year-old laptop:

Running c3
Program linked to executable 'related'.
c3:

        Processing time (w/o IO): 19ms
        total: 0.07s memory: 26344k
        Processing time (w/o IO): 19ms
        total: 0.07s memory: 26420k
        Processing time (w/o IO): 20ms
        total: 0.07s memory: 26356k
        Processing time (w/o IO): 20ms
        total: 0.07s memory: 26340k
        Processing time (w/o IO): 20ms
        total: 0.07s memory: 26280k
        Processing time (w/o IO): 19ms
        total: 0.07s memory: 26292k
        Processing time (w/o IO): 19ms
        total: 0.08s memory: 26408k
        Processing time (w/o IO): 20ms
        total: 0.08s memory: 26288k
        Processing time (w/o IO): 19ms
        total: 0.07s memory: 26276k
        Processing time (w/o IO): 19ms
        total: 0.07s memory: 26272k
Checking output
Success: related_posts_c3.json is valid!

Docker is untested. I just noticed there's separate Dockerfiles now, I'd be willing to test with that, should I make one?