Add comple-time checked const contructors for URI types

[SergioBenitez]

At present, the only way to construct an Origin or similar directly is through fallible means: parse(). For statically known data, it would be very convenient to have a compile-time checked variant:

impl<'a> Origin<'a> {
    // When used in a `const` context, fails at compile-time when `path` or
    // `query` are invalid `Origin` parts.
    pub const fn new(path: &'a str, query: &'a str) -> Origin<'a> {
        ...
    }
}

We can then provide an origin!() macro that forces a const context:

macro_rules! origin {
    ($path:lit, $query:lit) => ({
        static ORIGIN: Origin<'static = Origin::new($path, $query);
        ORIGIN
    })
}

Alternatively, we can make uri!() smarter so that it allows either Origin or Absolute URIs as the first argument and makes the route part optional. Then, the following would just work:

let _: Absolute<'static> = uri!("http://rocket.rs/foo");
let _: Origin<'static> = uri!("/foo/bar");

let _: Absolute<'static> = uri!("http://rocket.rs/foo", some_route: a, b, c);
let _: Origin<'static> = uri!("/foo/bar", some_route: d, e, f);

cc #998

[SergioBenitez]

Let's just roll this into #998.