Easier punctuation and keyword parsing

[dtolnay]

Currently parsing punctuation looks like syn!(ShlEq) and parsing a keyword looks like syn!(Default_). I would much rather write punct!(<<=) and keyword!(default) instead -- and have that produce the correct struct type without me having to look up its name.

[dtolnay]

As part of this, let's also explore what it would look like to use type macros for token types, rather than giving them names like ShlEq and Default_. Type macros have been stable since 1.13.

ItemExternCrate {
    pub attrs: Vec<Attribute>,
    pub vis: Visibility,
    pub extern_token: Token![extern],
    pub crate_token: Token![crate],
    pub ident: Ident,
    pub rename: Option<(Token![as], Ident)>,
    pub semi_token: Token![;],
}

[dtolnay]

Once the user is no longer typing the token names, will probably want to rename the various token structs to prefix the name, like ShlEq -> TokenShlEq like we do for items and exprs. This will avoid conflicts with prelude types like Default and syn types like Type (after we rename Ty -> Type).

[dtolnay]

Implemented in #241.