Fix clippy issues.

[jsuereth]

It looks like some new checks/improvements are made of this variety:

• Use y.clone_into(x) instead of x = y.to_owned()
• Don't borrow unless you need to.
• Use character literals instead of string literals of size 1

[jsuereth]

Another quick note: I disabled some clone vs. to_owned checks in the comment filter, as we would need a mutable-borrow version of trim for no great reason. We COULD create such a function, but I think the current performance hint is minimal for codegen and we can address that in the future.