Add syntax extension ary![expr, ..N]

`ary![expr, ..N]` expands to an array by repeating `expr` N times. This is
in contrast to `[expr, ..N]` which Copies `expr` N times.

Sample usage:

    let x = ary![None::<Vec<u8>>, ..10];

The one limitation is the count needs to be an integral literal, it
cannot be a static or a compile-time expression.

Related to #5244, but is more flexible.