Implement 'url!(..)' macro

[frewsxcv]

Fixes #136

[frewsxcv]

I preemptively opened this because I have a couple questions:

1. How should tests be structured/organized for compiler plugins?
2. Should the url crate depend on url_plugin? Wouldn't that create a cyclic dependency?

[Manishearth]

I think for publishing to work you need to have a version dep here too. Not sure

[SimonSapin]

To answer questions:

1. I don’t think many tests are needed here, one to check that the plugin doesn’t panic and returns something sensible should be enough. It’d have to be in a separate crate so you can use the plugin. Cargo will compile a separate crate (run a separate rustc --test invocation) if you have a file named e.g. plugin/tests/smoke_test.rs, so you don’t need another Cargo.toml file.
2. No it shouldn’t, why? Yes it would.

The code looks ok but (sorry I didn’t mention this earlier) I wonder if this plugin should be here rather than in Servo. We can move it back if someone asks, but in the meantime I don’t know if anyone else would use it. Meanwhile, this code uses unstable compiler internals (as plugins do) and rust-url is sort of expected to work on Rust Nightly, while Rust-in-Servo is updated less frequently.

To put it more bluntly, moving it to Servo is likely less future work for me :]

[Manishearth]

I wonder if this plugin should be here rather than in Servo

We don't need to ship this crate as part of rust-url. It can be published separately and used only by Servo.

[SimonSapin]

I’m suggesting having this in the servo/servo repository rather than this one, where the Rust version used on CI is updated less frequently. This is not about publishing on crates.io.

[frewsxcv]

Suggestions on where it should live in Servo?

[SimonSapin]

The plugins crate?

[frewsxcv]

servo/servo#8622