Remove dependency on Syntex

[dtolnay]

We depend on Syntex for three things:

• parsing
• quasi
• aster

I wrote a Rust parser https://github.com/dtolnay/syn and quasi-quoter https://github.com/dtolnay/quote which together build in 8.1 seconds (including all their dependencies). Syntex (including quasi and aster and dependencies) builds in 65 seconds so it is a decent improvement.

I am going to start porting serde_codegen over to these. For now serde_codegen will still need Syntex as a shim but serde_derive can disable it with a feature flag and use the new approach directly.

This will eliminate the biggest advantage of serde_macros over serde_derive - compile time. Once serde_derive is off of Syntex I propose that we stop supporting serde_macros.

[dtolnay]

To clarify the "for now": serde_codegen will need Syntex until either of the following happens:

• Macros 1.1 is stabilized and we no longer need to parse all of Rust in a build script Tracking issue for "Macros 1.1" (RFC #1681) rust-lang/rust#35900
• We can parse all of Rust in a build script without Syntex Add the rest of Rust syntax behind a feature flag (off by default) dtolnay/syn#4

Until one of these things happens, serde_derive can depend on serde_codegen but flag off the Syntex dependency.

[dtolnay]

In progress in #548, though there is a lot of work left.

[dtolnay]

Fixed in #548 for serde_derive.

Closing because I don't currently plan to do this for serde_codegen, but we can reopen if plans change.