script: Wrap remaining unsafe code and enable `unsafe_op_in_unsafe_fn`

[mrobinson]

This is last step toward enabling the default rustc unsafe_op_in_unsafe_fn warning for the script crate. It wraps the remaining unsafe code in unsafe {} and removes the line disabling this warning from script's Cargo.toml. In addition, two variables are renamed from v to something slightly more descriptive.

Testing: This should not change behavior so is covered by existing tests.