Allow non-`Global` allocators in `vec![in Alloc; 1, 2, 3]`

[jcallen]

Currently, the vec! macro effectively requires that the allocator always be Global when using the vec![in Alloc; 1, 2, 3] form, because it ends up calling Box::<[_]>::into_vec() (which is Box::<[_], Global>::into_vec()) instead of Box::<[_], _>::into_vec().