ENH: Add SerialBagging classes

[richford]

This PR creates a private module with serial versions of the sklearn.ensemble bagging estimators.

This is needed mostly because I struggle with nested parallelism when using dask.distruted as a parallel backend. In order to expose parallelism at the base estimator level (which is where it usually does the most good for me), I've found that I need meta-estimators and cross-validation functions to be run in serial. This is unnecessary when using the default joblib backends, but becomes necessary when using dask.distributed. If someone can tell me how to fully exploit nested
parallelism when using a dask.distributed backend, I will gladly remove this private module.