Allow inferring enum variant types with `_::Variant` syntax

I’ve recently been bouncing between Swift and Rust and have found the shorthand .variant syntax to be very useful. Having something similar in Rust would be very nice to have.
As initially suggested in #2830, the proposal is to allow explicitly inferring types for enum variants by using a _:: prefix.

Examples

enum CompassPoint {
    North,
    South,
    East,
    West
}

let mut direction = CompassPoint::West;
direction = _::East;

match point {
  _::East => { ... }
  _::West => { ... }
  _ => { ... }
}

fn function(cp: CompassPoint) {}

let direction = _::West;
function(direction);

Read this issue and the proposal in #2830 first, then compare the requested _::Variant behavior across the assignment, match, and function-call examples. Done means the RFC-level syntax and semantics are specified for all three contexts, with unresolved design questions addressed.