Improve compile times

[alecmocatta]

~Abysmal since #51.~

• [x] ~Avoid gigantic types (which require upping type_length_limit).~
• [ ] Avoid closures that I think are monomorphised for parameters that aren't relevant?
  • [ ] Some uses of poll_fn remain
• [ ] Try out cargo llvm-lines.

[alecmocatta]

#80 fixed most of the fallout from #51, so type_length_limit no longer needs upping and compile times are back to bearable.