Make [deny(unsafe_op_in_unsafe_fn)] optional

[thedataking]

We should not emit an extra unsafe block around most function bodies.
The c2rust output is less verbose and easier to consume by TRACTOR tools if we silence the lint instead.

[Rua]

In the 2027 edition that's planned to become a hard error, probably no longer allowable. What happens then?

[thedataking]

internal discussion:

• makes sense to leave machinery to generate these unsafe blocks (not reverted)
• turning off unsafe blocks for now
• could add option to turn on (don't want to fully snapshot test resulting code with unsafe blocks)