chore: removed needless mut references (clippy::needless_pass_by_ref_mut).

[Iluvmagick]

I've first noticed this in the DSL Builder struct, where the vec function takes a &mut self, while not even using self.
This prompted me to run cargo clippy -- -W clippy::needless_pass_by_ref_mut, and fix everything I could find.

[jonathanpwang]

We prefer not to change these for security reasons. Some of the &mut don't need to be there but are warnings of underlying mutations at the compiler level.