Remove usage of percent_decode_str for decoding query string key-value pairs (#1417)

* Use `form_urlencoding::parse` over `serde_urlencoded::from_str`, this removes our dependency on `serde_urlencoded`.

* Remove `percent_encoding::percent_decode_str` where possible.

* Contract `generateParsePercentEncodedStrFn` and its children into one function and make it optionally apply percent encoding.

* Add `impl From<std::convert::Infallible> for RequestRejection` to remove codegen branch.