Update documentation and make method names more expressive.

[Alexhuszagh]

This ensures any ambiguity from the method names would not cause potential issues among maintainers. There's 2 abstractions that contain 193/250 of the unsafe expressions and everything is clearly documented and the safety requirements are clearly documented and effectively every call of these unsafe methods is made within the trait/struct.

For example, in Bigint, the majority of the "unsafety" are methods like push_unchecked, pop_unchecked, and extend_unchecked, which are only called by try_push, pop, and try_extend, respectively. The same logic extends for essentially the entire implementation.

Closes #100
Closes #138