747: Base v0 mangling grammar r=philberty a=CohenArthur

This PR adds base functions to deal with the v0 mangling grammar, [found here](https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html#syntax-of-mangled-names).

I have a few questions regarding this implementation:
1/ Is there any existing implementation for the base62 algorithm used here? This is directly adapted from [rustc's base_n module](https://github.com/rust-lang/rust/blob/6f53ddfa74ac3c10ceb63ad4a7a9c95e55853c87/compiler/rustc_data_structures/src/base_n.rs#L16) which I'm assuming is relatively standard and might already exist in the compiler. I haven't been able to find it however.
2/ gccrs cannot yet deal with unicode identifiers, as pointed out by `@bjorn3` in #418. This means that a big chunk of the `v0_add_identifier` implementation is missing. Should it be added in this PR too?
3/ As mentionned in zulip, it would be great to be able to create unit tests for this piece of code. It would be quite easy to generate a bunch of base62 strings and ensure that the algorithm here matches with them.

Co-authored-by: CohenArthur <[email protected]>