Enable the deny(unsafe_op_in_unsafe_fn) lint everywhere in librustzcash we define unsafe functions

[daira]

Similar to Electric-Coin-Company/zcash-light-client-ffi#65 :

A side-effect of declaring a function unsafe is that unsafe operations in its body are no longer required to be in unsafe blocks (see Rust RFC 2585 for a proposal to change this, not yet adopted). Ideally, we should enable the deny(unsafe_op_in_unsafe_fn) lint everywhere we define unsafe functions, since the style we write FFI functions in is mostly already consistent with that lint.

[daira]

This can be enabled crate-wide by adding

[build]
rustflags = ["-Dunsafe_op_in_unsafe_fn"]

in .cargo/config.

[daira]

This should also update all extern functions with safety requirements on callers to unsafe extern.