Stop using the `quote_` macros

[SergioBenitez]

An issue/PR removing the quote_ macros from rustc is underway (rust-lang/rust#51285, rust-lang/rust#46849). Rocket uses quite a few of these macros in a fairly fundamental way, so this is a fairly large breaking change for Rocket. A workaround is needed.

[eddyb]

Ideally a full transition to proc_macro + syn + quote should be done, to avoid further trouble when the existing plugin API will be removed.

[SergioBenitez]

That's the hope, @eddyb, but it's a serious undertaking that I was hoping to postpone for another month.

[SergioBenitez]

Let's track this in #693.