Mark 'match' as constant expression, if appropriate

[BenWiederhake]

Small-ish reproducing example (Play):

const N: u32 = 4;
const SPECIAL_RULE: &'static str = match N {
    3 => "Note: Must remote two cards before play.",
    6 => "Note: Must remote four cards before play.",
    _ => "No further notes.",
};

fn main() {
    println!("{}", SPECIAL_RULE);
}

Expected / desired behavior of the compiler:
Compiles without warning or error.
More generally: match expressions are allowed in definitions of constants, if the matching expression, all clauses, and all value expressions are constant. (Or something like that.)

Actual behavior of the compiler:
Compilation error [E0019]: constant contains unimplemented expression type.
More generally: match does not seem to be permitted at all.

Occurrence in bugtracker:
I have found no other instance in Google, and GitHub only lists #11411, which is the same error with a different syntax element.

Btw, if is not permitted either, so const MAGIC: u32 = if N == 4 { 42 } else { 1337 }; fails, too.

[BenWiederhake]

Closing on behalf of rust-lang/rfcs#322