Add pattern matching for character literals

[DeSevilla]

Can now define functions and case statements that pattern-match on character literals, e.g. case c of '\n' -> 0. Adds a few tests for this functionality.

[DeSevilla]

I'm attempting to solve the failing CI/build, but when I run stack test on the current main branch, before any of my commits, I get all the same failed tests. Actually, I get a few more on my machine, mostly because the solver finds some equivalent but slightly different counterexamples for negative tests than are in the golden files. As far as I can tell, my commit doesn't affect any of the existing tests. I'm not sure what I can do about that.

[DeSevilla]

On second look, it seems like all 8 of the tests that failed in CI are the ones listed here as ongoing failures: #294

[jacobpake]

I get a few more on my machine, mostly because the solver finds some equivalent but slightly different counterexamples for negative tests than are in the golden files.

This happens between Z3 versions, if you use 4.15.4 it should match up.

And yes, unfortunately there are 8 failing tests currently in the main branch! Consider it passing as long as that number doesn't increase.