Update syn to 0.12 and quote to 0.4

[Eijebong]

This change is 

[SimonSapin]

In macros/lib.rs, could we convert between proc_macro::TokenStream and proc_macro2::TokenStream without going explicitly through String? That way enabling the proc_macro2/nightly cargo feature would preserve span information and produce better error message if something fails to compile.

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Reviewed 5 of 5 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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build/match_byte.rs, line 20 at r1 (raw file):

    let mut source = String::new();
    File::open(from).unwrap().read_to_string(&mut source).unwrap();
    let tts = syn::parse_file(&source).expect("Parsing rules.rs module");

Rename tts (token trees) to something like ast?

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Comments from Reviewable

[SimonSapin]

Never mind, I forgot that these conversions to/from strings are in precedural-masquerade, refactoring that would be more involved than I thought.

@bors-servo r+

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Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[bors-servo]

📌 Commit a244c3c has been approved by SimonSapin

[bors-servo]

⌛ Testing commit a244c3c with merge 39a4706...

[bors-servo]

☀️ Test successful - status-travis
Approved by: SimonSapin
Pushing 39a4706 to master...