Proposal: split out types into standalone package

[ocramz]

While building xml-conduit-microlens (https://hackage.haskell.org/package/xml-conduit-microlens) I noticed that the cold-build time is still pretty large due to the transitive dependency on conduit-extra.
Since some types have been redefined (for the better) wrt xml-types and other libraries such as xml-lens rely on them too, would it be an option to have a types-only package for XML that downstream libraries depend on?

It would have to depend on blaze still to have the ToMarkup instances, but not on conduit and friends.