Improve the vec![...] macro with UFCS.

Improve the vec![...] macro with UFCS.

There are two limitations to the macro that this addresses:
1. the expected type is not propagated, coercions don't trigger
2. references inside element expressions don't outlive the `Vec`

Both of these limitations are caused by the block in the
macro expansion, previously needed to trigger a coercion
from `Box<[T; N]>` to `Box<[T]>`, now possible with UFCS.