Monomorphize code via --monomorphize

[N1ark]

Not anywhere near done, but I'll probably work on this in parallel as I need to cover more code -- for now I'm doing the bare minimum, and opening the draft PR to give some visibility.

Add monomorphisation, by applying an algorithm similar to what rustc does:

1. Find roots (public monomorphic functions)
2. Generate monomorphic items:
   1. Find uses of other items within the item (e.g. the function's body, the struct's fields...)
   2. For all used items that are polymorphic, make a monomorphic copy with the generic arguments substituted, if one doesn't exist already.
   3. Substitute these uses with the monomorphic version, which is added to the discovered items.
3. Remove all polymorphic items; if they were used anywhere, they have already been monomorphized and substituted.

Note a consequence of this algorithm is that it does dead code removal of polymorphic items; any unused polymorphic item will have 0 monomorphic items created, and will thus be removed.

Added a --monomorphize flag to do this.

[msprotz]

This is super great and something that I will need at some point for Eurydice, too.

Just a heads-up that the main maintainer of Charon, @Nadrieril, will be back online on Monday to help with the review. Thank you!