Just delete a bunch of string_append pattern things

It's so broken, it's not worth even trying to fix. Right now it just generates invalid
functions for otherwise functional mappings. Instead, turn any match arm

x ^ ... ^ y => <exp>

into

_ => exit()

String mappings have generated a special non-executable effect for a while, so this shouldn't
break anything in practice.

On this plus side, generated string append code represented the majority of generated OCaml
for RISC-V, so this fixes the compilation time issues

Author: Alasdair