Replace the ast::Mac_ enum with a struct

[Manishearth]

https://github.com/rust-lang/rust/blob/master/src/libsyntax/ast.rs#L1137

pub enum Mac_ {
    MacInvocTT(Path, Vec<TokenTree>, SyntaxContext),   // new macro-invocation
}

this should instead be something like

pub struct MacroInvoke {
    path: Path,
    tts: Vec<TokenTree>,
    context: SyntaxContext,
}

We'd need to update all type and other errors.

I'm willing to mentor this.

Please leave a comment if you're working on this and let me know if you need any help.

[apasel422]

I'm working on this.

[Manishearth]

Awesome!

[Manishearth]

Note: You may want to base it on top of #28526, and convert the complicated "and_then + destructuring match" to a simpler "move MacroInvoke out of P<T> via and_then and then use fields throughout the function"