Forbid unsafe code in each crate

[kevaundray]

Problem

Currently, we do not use any unsafe code and its advantageous to keep it this way so that we rely on the static analysis done by the compiler.

Solution

We can signal/assert that we have no unsafe code by adding #![forbid(unsafe_code)] to the crate root of each crate.

The crate root for a library is lib.rs and the crate root for a binary is main.rs.

We only have one binary in the nargo crate, though since this file is doing nothing, we only need to modify the lib.rs files.

Alternatives considered

Additional context