paddle: Take any kind of `str` reference input, not just `&str`

Use `AsRef<str>` instead of `&str` to offer a more flexible interface.
We need this because `url::form_urlencoded::parse()` gives us an
iterator of `(Cow<_, str>, Cow<_, str>)`, and we want to pass that into
`verify_signature()`.

Also change `key.len()` and `value.len()` to `.chars().count()` because
I was having a hard time getting the `len()` method from a trait (`str`
doesn't implement `ExactSizeIterator`), and I learned this:

> This length is in bytes, not chars or graphemes. In other words, it
> may not be what a human considers the length of the string.

(https://doc.rust-lang.org/std/primitive.str.html#method.len)

Also:
https://stackoverflow.com/questions/46290655/get-the-string-length-in-characters-in-rust/46290728#46290728

I assume the PHP serializer uses character count instead of byte length.

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