REF/INT: concat blocks of same type with preserving block type
Related to https://github.com/pandas-dev/pandas/issues/17283

Goal is to get `pd.concat([list of series]` working with Series with external block type. Currently the values are always converted to arrays, concatenated, and converted back to block with block type inference.

This is a proof-of-concept to check whether something like this would be approved. Tests should still break because I didn't specialize the `concat_same_type` for categorical data.