Enable unsafe_block_in_unsafe_fn feature

[phil-opp]

See rust-lang/rust#71668

[josephlr]

We will have to wait for that lint to stabilize, right? Otherwise it seems like it would be hard to keep things warning free on both stable and nightly.

[phil-opp]

We could allow the unused_unsafe lint on stable to avoid the warnings.

[josephlr]

#![deny(unsafe_op_in_unsafe_fn)] is stable, see rust-lang/rust#79208!

So we can just add this whenever