Implement fold

[dtolnay]

Here is how libsyntax does it: https://github.com/rust-lang/rust/blob/master/src/libsyntax/fold.rs

cc @SimonSapin

[SimonSapin]

Why a separate crate?

[dtolnay]

I am hesitant to pile more features into syn because if an optional feature requires a breaking change, even people not using that feature suffer from the version bump. With a relatively stable core in syn and an ecosystem of supporting libraries around it, you are only affected by breaking changes to pieces that you use.

I guess fold is core enough and coupled so closely with everything else in syn that the scenario of breaking fold without breaking syn seems unlikely, so you're right it could make sense to implement here rather than a separate crate.