Reconsider usages of unsafe

[aochagavia]

Resolvo currently has 13 unsafe blocks. They are related to two things:

• Getting stuff from a slice / vec without bound checks;
• Implementing FrozenCopyMap.

Since this is a pretty foundational library, I think we should strive to either avoid using unsafe entirely or otherwise make sure to have the appropriate infrastructure to prevent errors (e.g. running tests under miri, fuzzing, etc).

@baszalmstra any thoughts on this? I assume the unsafe that has been introduced was due to performance optimizations. Did it provide a measurable improvement?