Allow `_ @ CONST` in match arms

The following is currently not valid:

const CONST: u32 = 1;

fn main() {
    let x = 2;
    match x {
	    _ @ CONST => { },
	    _ => { }
    }
}

error: pattern on wrong side of `@`
 --> src/main.rs:6:6
  |
6 |         _ @ CONST => { },
  |         -^^^-----
  |         |   |
  |         |   binding on the right, should be on the left
  |         pattern on the left, should be on the right
  |         help: switch the order: `CONST @ _`

Apparently because the grammar requires an identifier on the lhs of @. Replacing _ by _x makes the code compile.

Writing

    match x {
	    CONST => { },
	    _ => { }
    }

is ambiguous without a complete analysis of the surrounding module. CONST might be a variable name or a constant. The difference impacts which arm is taken.

On the other hand,

    match x {
	    _x @ CONST => { },
	    _ => { }
    }

is unambiguous but introduces the useless variable _x.

Therefore my proposal is as follows:

• Make

    match x {
	    _ @ CONST => { },
	    _ => { }
    }

valid syntax.

• Deprecate

    match x {
	    CONST => { },
	    _ => { }
    }

where CONST could either be a constant or a variable name in the next edition.

Start with the match-arm examples in the issue and analyze the pattern grammar and the ambiguity between constants and variable names. A complete proposal should define the accepted _ @ CONST syntax and the next-edition treatment of the ambiguous CONST form, then document the resulting behavior through the RFC process.