Reimplement StatusCode and HttpVersion using associated constants

[dtolnay]

The associated constants feature is almost ready. rust-lang/rust#29646

Something like:

pub struct StatusCode(pub u16);

impl StatusCode {
    pub const OK: StatusCode = StatusCode(200);
    /* ... */
}

A few advantages:

• We can easily add new codes as the relevant standards change. Currently adding a new status code would involve a breaking change to hyper.
• Resolves the ambiguity around whether StatusCode::Ok is equal to StatusCode::Unregistered(200).
• The size of a status code goes from 4 bytes to 2 bytes which is what it should be.

[seanmonstar]

Blocked by rust-lang/rust#29646

[seanmonstar]

This was done in the http crate. The types there will replace the ones here in hyper in v0.12.