Refactor API

[yoshuawuyts]

This is a whole set of related changes, addressing most of what was laid out in #336. This disables a lot of work, but sets us up to carefully reintroduce later.

The biggest changes are that we have several core constructs:

• Server is the main application struct (formerly called App).
• Endpoint (trait) is the endpoint people work with.
• Request is what is is passed to Endpoint and represents an incoming request. (formerly called Context).
• Response is what is returned from Endpoint and represents an outgoing response.

Both Request and Response operate with AsyncRead, and have shorthand methods to send and receive json, forms, querystrings, etc.

I believe this should dramatically streamline the experience of using Tide, and overall make writing HTTP services a lot nicer!

Examples

echo server

let app = tide::new();
app.at("/echo").get(|req| async move { req });
app.listen("localhost:8080").await?;

hello world

let app = tide::new();
app.at("/echo").get(|_| async move { "hello world" });
app.listen("localhost:8080").await?;

Screenshot

[yoshuawuyts]

Tests pass; has also been reviewed by a few folks. Going to go ahead and merge!