BUG: Concat multiple different ExtensionArray types
xref https://github.com/pandas-dev/pandas/issues/22994

This builds on https://github.com/pandas-dev/pandas/pull/22996, since we need hashing.

2a1660c77e8ad0c28485c6d81227ac9d6b6de4f2 is the relevant commit.

Sparse tests will fail for now. I'll revisit once SparseArray is in. 