Support bare wildcard constraints

Cabal has long had a syntax for wildcard constraints: `foo ==1.*` desugars into `foo >=1 && <2`. I was surprised to discover (via https://github.com/sol/hpack/issues/201) that bare wildcards are not supported: `foo ==*` is a parse error. I would expect it to be the same as `>=0` (or `-any`). Is this something that could be added to Cabal?