Recommend the systematic use of lint unsafe_op_in_unsafe_fn for crates with unsafe code

[polazarus]

With version 1.52, Rust supports a new lint to encourage Rust developers to mark unsafe code everywhere even in unsafe function.

In a security conscious development the global lint #![deny(unsafe_op_in_unsafe_fn)] should be present at the top of the crate, if the crate includes unsafe code.

Notable uses:

• allow unsafe as a pure marker for should-not-be-used-from-Rust function which are not literally unsafe,
• distinguish unsafe pointer-dereferencing code from safe wrapping code in extern "C" function definition.