unsafe attribute and api.rs

[wks]

Clippy gives teh not_unsafe_ptr_arg_deref lint warning if a function dereferences a raw pointer but is not labelled as unsafe. We used to suppress this warning was because labelling a function unsafe automatically grants it rights to use any unsafe operation inside the function. I deemed that a greater danger because we then won't be able to identify real unsafe sections within an unsafe function.

We now set #![warn(unsafe_op_in_unsafe_fn)] for the whole lib. It makes it necessary to use the unsafe { ... } block even within unsafe functions. We now explicitly label functions as unsafe if they deference raw pointers. But we set
#![allow(clippy::missing_safety_doc)] on the api.rs module because the reason for functions to be unsafe in that module is clearly because they are used by C with raw pointers.