Add string comparison to LLVM backend

[jiribenes]

I'd like to have a function compareString: (String, String) => Ordering for #561.
See #561's changes in effekt.effekt for the kind of function we need, then figure out a way to support it on the LLVM backend. :)

Should it be native (somewhere in C)? Should it be written in Effekt itself? No clue.
I think it should be possible and reasonably easy to write this function directly in Effekt, at least if the comparison is byte-wise.

[jiribenes]

At the Effekt Working Group Meeting ™️, we talked about this: perhaps having a comparison returning -1, 0, 1 on native byte strings would make sense, then we can just set compareString to use s1.toByteString.compare(s2.toByteString)