Implement valuable-http, a bridge between the http crate and valuable

[carllerche]

Until Valuable reaches 1.0, http will most likely not depend on valuable, so we should provide a bridge. This crate would provide wrappers for types in http that implement Valuable.

Then, users can do:

use valuable::Valuable;
use valuable_http::Request;

Request::from(my_request).as_value()

One open question: what argument should the Request type take?

• http::Request
• &http::Request
• impl AsRef<http::Request>

[aatifsyed]

I'll give this a go :) - a crate in this project or elsewhere?