Redesign compiler diagnostic logging

[emberian]

The interface in syntax::diagnostic is super janky and causes a lot of problems with code higher up in the stack (it prevents anything from being sendable, for example). Rework it to use idiomatic traits, include a JSON output mode, and add the ability to work in #2092.

[emberian]

(I'm actively working on this)

[huonw]

cc #3533.

[emberian]

#3533 is not in scope for this.

[emberian]

I'm not actively working on this.

[ahmedcharles]

@cmr Did you have any code or suggestions about changes that should be made beyond the comments above?

[emberian]

@ahmedcharles I spent a few days working on this and then got busy with school. I started with rewriting syntax::diagnostic to look like what I thought it should, and then got lost in refactoring the compiler to compensate.

[steveklabnik]

Triage: @cmr what do you think of this today?

[emberian]

@steveklabnik I think this is still desirable, even more so for tools like Cargo to "understand" what rustc is outputting.