Add of_string helpers

[rusty-key]

It might be useful to be able to parse paths into data, for example for universal routing.

Currently, it's not exposed:

For Melange the ppx will emit:

val http_method : _ t -> [ `DELETE | `GET | `POST | `PUT ]
val href : 'a t -> string
val body : 'a t -> string option
val decode_response : 'a t -> Fetch.Response.t -> 'a Js.Promise.t