refactor: enable safer unsafe clippy lints

[TheBestTvarynka]

Checks for the result of a &self-taking as_ptr being cast to a mutable pointer.

https://rust-lang.github.io/rust-clippy/stable/index.html#as_ptr_cast_mut

Checks for casts of a function pointer to any integer type.

https://rust-lang.github.io/rust-clippy/stable/index.html#fn_to_numeric_cast_any

Checks for as casts between raw pointers that change their constness, namely *const T to *mut T and *mut T to *const T.

https://rust-lang.github.io/rust-clippy/stable/index.html#ptr_cast_constness

Checks for // SAFETY: comments on safe code.

https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_safety_comment

Checks for the doc comments of publicly visible unsafe functions and warns if there is no # Safety section.

https://rust-lang.github.io/rust-clippy/stable/index.html#missing_safety_doc

[CBenoit]

LGTM