Handle macro error diagnostics

[alexheretic]

Produce useful diagnostics from macro messages.

Previously the file reference for macro diagnostics were coming out as something like ./<vec macros>. This meant the error doesn't properly show up at the site in source, and suggestions are not usable.

Test Case

fn main() {
    let mut out = String::new();
    write!(out, "{}", 123);
}

Before

This is quite annoying in large code bases, where is this error?!

After

Resolves #183
Resolves #638

[alexheretic]

Btw test success is waiting on rust-lang/rustfmt#2748

[alexheretic]

Ok I just included rustfmt update

[nrc]

This is probably somewhere rustc could do better so we don't need to hack around it here, but I don't know the constraints there.

Thanks for the PR!