Use `#![deny(unsafe_op_in_unsafe_fn)]` in the CAPI

[cschwan]

An unsafe function is automatically an unsafe block, which is unfortunate because these are two different concepts: https://rust-lang.github.io/rfcs/2585-unsafe-block-in-unsafe-fn.html. With unsafe_op_in_unsafe_fn they can be disentangled.

[alecandido]

This is fascinating[] to know (I never paid too much attention to unsafe), but I wouldn't say it's unfortunate*.

As the RFC explains very well, unsafe to call and an unsafe block are strictly related, because the block it's exactly what allows that kind of call (and similar unsafe operations, of course).

[*]: my intrusive spell checker suggested that "really interesting" was a bit poor...

[cschwan]

Implemented in commit 4d74a051800a405d1056cb4368b340783dfa60c2.