Fixed top-level util import issue

[philippjfr]

As pointed out in #1134 (comment) it turns out my previous fix wasn't quite sufficient, the wildcard import from hv.core.spaces was overriding hv.util with hv.core.util. Ideally we should get rid of all wildcard imports to avoid issues like this.

[PeterDSteinberg]

I agree on getting rid of wildcard imports, or limiting their usage and using them only when __all__ has been defined.

[philippjfr]

Right, I think we can easily replace the remaining wildcard import in the top-level __init__, the only one I would leave is for the element module, which does define __all__ and would be too much of a pain to list out explicitly.

[philippjfr]

Just checked and all other imports define __all__. Ready to merge.

[jlstevens]

Great!