Deprecate legacy API in favor of traversable API (files)

Following from !76 (merged) and this comment, I'd like to explore what the implementation would look like with only the traversable API. In that comment, I wrote:

I've started stripping out [the legacy] functionality in the feature/drop-legacy-api branch. Progress was good, but I got stumped on Traversable.open() for zip files, which doesn't support text-decoding. This leads me to believe that's a feature zipp should support. This also suggests the Traversable API should include .open().