Rewrite the AST to be a bit more user-friendly

[alexcrichton]

This commit is a relatively large rewrite of the AST that syn exposes. The
main change is to expose enums-of-structs rather than
enums-with-huge-tuple-variants. The best example of this is ItemKind::Fn which
changed from:

enum ItemKind {
    Fn(Box<FnDecl>, Unsafety, Constness, Option<Abi>, Generics, Box<Block>),
    ...
}

to

enum ItemKind {
    Fn(ItemFn),
    ...
}

struct ItemFn {
    decl: Box<FnDecl>,
    unsafety: Unsafety,
    constness: Constness,
    abi: Option<Abi>,
    generics: Generics,
    block: Box<Block>,
}

This change serves a few purposes:

• It's now much easier to add fields to each variant of the ast, ast struct
  fields tend to be "by default ignored" in most contexts.
• It's much easier to document what each field is, as each field can have
  dedicated documentation.
• There's now canonicalized names for each field (the name of the field) which
  can help match match statements more consistent across a codebase.

A downside of this representation is that it can be a little more verbose to
work with in match statements and during constructions. Overall though I'd
feel at least that the readability improved significantly despite the extra
words required to do various operations.

Closes #136

[alexcrichton]

Note that tons of the names here are bikesheddable, I wouldn't mind tweaking anything!

[dtolnay]

Amazing!

How would you feel about putting these tuple variants in modules? For example expr::IfLet instead of ExprIfLet. That would help avoid the even-worse wall of types when you open the documentation. I'll merge this quickly and we can follow up separately if you agree.

[alexcrichton]

Sounds great to me!

[alexcrichton]

Ah so I've discovered that if we do IfLet as a struct name then you can't import both ExprKind::* and expr::* unfortunately, leaving us with a situation that chooses between a number of the following in a match statement:

// currently proposed
IfLet(ExprIfLet { ... }) => 

// Drop "Expr" prefix
ExprKind::IfLet(IfLet { ... }) => 
// or
IfLet(expr::IfLet { ... }) => 

// another random prefix?
IfLet(EIfLet { ... }) => 

[dtolnay]

Let's keep the current approach for enums.

The build failure looks like TyPtr not printing "const".

[alexcrichton]

Alright I think that'll do it!

[alexcrichton]

Ah looks like I missed a few things when testing in 1.12.1, but now it's getting into the realm of "may not be super easy to fix". Looks like this'd bump to 1.15.0+

[dtolnay]

That's fine. I'll remove those Travis builds.

[dtolnay]

Up to Rust 1.15.1 in d9e2dc9.

[alexcrichton]

👍