Adapt for removal of old interfaces in libsyntax
Adapt for removal of old interfaces in libsyntax

https://github.com/rust-lang/rust/commit/9108fb7bae11f18715d971eeae1e5ca84662e1ee
dropped the `map()` method (among other things) -- so we need to adapt
our code.

This change should be backwards-compatible.

serde-rs/serde,erickt/serde,oli-obk/rust-serde