Fix owned string usage

Hi!

Recently [`~"string"` literals have been removed](https://github.com/mozilla/rust/pull/13877) from rust master, being replaced by `"string".to_owned()`.

The first commit is just an automatic conversion from the first form to the later, and by itself already makes the library compilable again on rust-nighly.

The second one removes multiple uses of `"string".to_owned()` that were unnecessary: `"string"` evaluates to `&'static str`, which is enough most of the time, and doesn't need heap allocation. It probably won't make any significant difference on speed or memory usage, but avoiding heap allocation when possible seems to be the current best practice for idiomatic rust.