Pass input by ref

[Jake-Shadle]

to_string does not need to take the serialization input by value, changing it to pass by reference
aligns with how most other serde crates work.

Also adds a serde test that is an example of the use case that spurred this change.

[Arnavion]

The test will pass even without this change, because T: Serialize already implies &T: Serialize

[nox]

That's a breaking change, I'll keep it in mind next time I feel like doing a breaking bump.