Clean up String handling

[birkenfeld]

Since Haskell doesn't use string slices, there's a lot of places with unnecessary .to_string() use, and functions with String instead of &str in the signature.

[tcr]

It just occured to me, I bet clippy would identify a ton of this excessive heap use. (One more reason to exclude parser-c-core from the root crate).

[birkenfeld]

I was kind of dreading running Clippy for now... 😄