Document all the public API

[seanmonstar]

We should add #![deny(missing_docs)] to the top of the h3 crate, and document all the types and methods that the lint catches.

[stammw]

We should also use this opportunity to reorder the modules so they are similar to h2 and hyper:

use std::*;

use external_crate::*

use crate::*;

/// Doc Foo
struct Foo {}

/// Doc Bar
enum Bar {}

// --- impl Foo
impl Foo {}

impl Baz for Foo {}

// --- impl Bar
impl Bar {}

impl Quux for Bar {}

[Ruben2424]

I think this can be closed. Or is something left to document?