100x slower performance on serde_derive (with recent serde and syn?)

I just tried to use watt with serde_derive to improve performance in a project of mine, only to find it got much slower running the derive macros. Just compiling the shim serde_derive crate is of course much faster.

For this snippet:
```rust
use serde_derive::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
pub struct Information {
pub a: Option,
pub b: Option,
pub c: Option,
pub d: Option,
pub e: Option,
pub f: Option,
pub g: Option,
}

fn main() {}
```

`cargo +nightly rustc -- -Z time-passes` gives me:
`time: 2.753; rss: 53MB -> 113MB ( +60MB) macro_expand_crate`

When using the normal serde_derive I get:
`time: 0.027; rss: 53MB -> 75MB ( +21MB) macro_expand_crate`

I am using a patched syn (rebased on master from jakobhellermann's): https://github.com/acshi/syn-watt/tree/watt-feature
And my patched serde is here: https://github.com/acshi/serde/tree/watt
The final shim crate I have is: https://github.com/acshi/serde_derive_wa

I am guessing this might have something to do with using the latest versions of syn and serde? For some reason I found I also had to patch serde_derive to output `r#try!` or I would get compilation errors because `try` is a reserved keyword.

Reproduce the provided Information snippet with the linked watt-feature syn, watt serde, and serde_derive_wa patches, using `cargo +nightly rustc -- -Z time-passes`. Compare the macro_expand_crate timings with normal serde_derive, then trace the relevant patched dependencies to identify the regression and confirm that the performance difference is resolved.

Written by the indexing model from the issue text.