Add variants to the type namespace

[brson]

Tracking rust-lang/rfcs#234

[nrc]

I have an implementation of this, just needs polish and a rebase...

[SimonSapin]

What is the replacement for serialize::json::List and serialize::json::Object, which currently are both enum variant and types, defined in the same module?

pub enum Json {
    I64(i64),
    U64(u64),
    F64(f64),
    String(String),
    Boolean(bool),
    List(List),
    Object(Object),
    Null,
}
pub type List = Vec<Json>;
pub type Object = TreeMap<String, Json>;

I use that pattern a lot in Servo:

pub struct QualifiedRule { ... }
pub struct AtRule { ... }
pub enum Rule {
    QualifiedRule(QualifiedRule),
    AtRule(AtRule),
}

[pnkfelix]

Assigning P-backcompat-lang, 1.0 milestone.

[nikomatsakis]

@SimonSapin indeed I use it a lot too, but hopefully we'll be able to get it back and better in future, when variant is an actual type.