Setup Safety Invariant Checks

[Alexhuszagh]

Rust allows the use of requiring annotations for unsafe code to document their safety invariants, as well as annotations for unsafe functions, which can help document and simplifying auditing and maintaining crates using unsafe code. Specifically, we should use the unsafe_op_in_unsafe_fn and ensure clippy is configured to check for these.

This will require:

1. allow(unused_unsafe)
2. warn(unsafe_op_in_unsafe_fn))
3. Setting up clippy

[Alexhuszagh]

Patched as of #9.