add Uri#into_builder() to easily modify a part of Uri

[gfx]

It is messy to modify a Uri instance with Uri::Builder. For example, if one wants to modify path_and_query, they should do something like this:

let uri: &Uri = req.uri();
let path_and_query: String = "...";
let parts = uri.clone().into_parts();
let modified_uri = Uri::builder()
    .scheme(parts.scheme.unwrap())
    .authority(parts.authority.unwrap())
    .path_and_query(path_and_query.as_str())
    .build()
    .unwrap();

With this PR, it will become:

let uri: &Uri = req.uri();
let path_and_query: String = "...";
let modified_uri = uri.into_builder()
    .path_and_query(path_and_query.as_str())
    .build()
    .unwrap();

It will let us focus on the concern.

What do you think of it?

[djc]

Unofficial review: this looks good to me; I think it's solving a real problem with a nicely compact API.

[robjtede]

Alternative design:
Builder::from_parts() / (impl From<Parts> for Builder) construction becomes Builder::from_parts(uri.to_parts()) / (Builder::from(uri.to_parts())).

Pros:

• equally comprehensible
• mirrors Uri::from_parts method
• (permits parts.into() where Builder is passed to fn)

Cons:

• uglier when chained
• requires Builder import

[reu]

Came looking for this! Any chance we can get this reviewed by a maintainer?