params.Entity preparations

This PR is a grab bag of changes aimed at isolating all the places where we pass a string to params.Entity and convert them into taking a names.Tag. 

This PR isn't complete, there is still more work to be done before params.Entity.Tag can be made into a names.Tag, but it is a good checkpoint as it gets rid of all the mustParseXXX methods added in a previous PR.
Never mind - this is not the Entity struct I was thinking of (state.api.params.Entity)