ANN: types for _create_storer
1) Annotate return type for _create_storer
2) Annotate other functions that we can reason about given 1
3) Fixups to address mypy warnings caused by 1 and 2
@jbrockmendel looking good. `Union["GenericFixed", "Table"]` used 4 times. not sure we have a policy for when we should use an alias, i.e. number of occurrences vs complexity of type expression. > used 4 times

In this case I think they're better kept as the Union.  In a perfect world id like to get separate functions one of which returns Table and the other returns GenericFixed, but i couldn't come up with a nice way of pre-checking i think i merged in an odd order, pls rebase merged lots of stuff, can you rebase just to be sure here. rebased+green k thanks