crucible-llvm: Refactor and export override pipe-fitting code

[langston-barrett]

The code in question converts between bitvectors and pointers so that it is easy and natural to specify overrides using bitvectors, while at runtime only LLVM pointers are used. The built-in overrides (e.g., for libc) all expect to be wrapped in this casting code. They're all exported, so to make them usable by downstream clients, we also need to export the code that does the pipe-fitting.

• [x] Factor it into its own module
• [x] Make it return errors instead of panicing
• [x] Possibly rename it
• [x] Add haddocks

[langston-barrett]

I have tested this using a non-Crucible-LLVM frontend, and it appears to work as intended.