Big Clippy sweep

[JSAbrahams]

• Mostly code style items
• Some clones here and there removed where not necessary.

This should also mean that clippy behaves at it should in future PR's (See #231),

Supposedly the behavior of the code-base has changed in some areas as we were using references where we should have been using values:

for pair in names.iter().cloned().zip_longest(elements.iter()) {
    match &pair {
        Both(name, exp) => {
            let expect = Expect::Type { name: name.clone() };
            let l_ty = Expected::new(&left.pos, &expect);
            constraints.push("tuple", &l_ty, &exp)
        }
}

However, no tests have caught this change in behaviour.