Implement byte-to-char casts for crucible-mir.

[sauclovian-g]

No description provided.

[RyanGlScott]

Thanks, @sauclovian-g! I can believe that this fixes the issue, but I would feel better if we added a regression test alongside it. Can you add this test case to crux-mir's test suite? I'm not too bothered about the exact location where we put the test—somewhere in test/conc_eval/num would work, for instance.