Introduce a Uri Builder

[seanmonstar]

This introduces a Builder type for manipulating and building up Uris. Instead of having mutators on Uri directly, which would need to return errors, instead this suggests that a user either use a Builder from the start, or to convert a Uri into via uri.into_builder().

An example looks like:

let uri = Uri::builder()
    .scheme("https")
    .authority("hyper.rs")
    .path_and_query("/")
    .build()
    .unwrap();

It currently only supports setting the 3 distinct parts, not any sub parts (like host or path), but I expect those could be added. This is however generic over HttpTryFrom, so you can easily build pieces with &str, or Bytes, etc.

It's only a start, but related to #206.

[bluetech]

Uri as described in the docs has 4 distinct forms:

request-target = origin-form
               / absolute-form
               / authority-form
               / asterisk-form

Maybe it makes sense to have 4 distinct builders, one for each form? (Though the asterisk form doesn't really need one).

[carllerche]

Maybe leave this fn off for now?

[carllerche]

Lgtm. Just needs docs

[carllerche]

Is this added for internal convenience or does it impact the public api somehow.

[seanmonstar]

Yea, it was internal convenience, allowing arg.http_try_into() instead of <T as HttpTryFrom<U>>::try_from(arg). Since the trait isn't public, and no one can import it, it exposes no public contract.

[carllerche]

Sounds good to me.

[carllerche]

Yeah, just needs docs + it looks like CI is busted for some reason.

[seanmonstar]

@carllerche took a while, but I finally cleaned this up.