Shield unsafe behind optimized feature

[vlopes11]

Implement a new feature optimized to shield all unsafe usage.

When this feature is on, unsafe is allowed

When this feature is off, we should introduce forbid_unsafe in the root of the library:
src/lib.rs: #![cfg_attr(not(feature = "optimized"), forbid(unsafe_code))]

This means that all functions that use unsafe will have to be reimplemented with their safer (and potentially slower) counterparts

[vlopes11]

Blocked by FuelLabs/fuel-types#5