HDF5 variable length strings

We'd like variable length HDF5 string support.  Sadly the Python options tend to go through NumPy, which has poor varlen string support.  `h5py` has a workaround. 

This only works through the Python layer at the moment.
This complication of the code was a performance thing, right? Probably deserves a comment noting that, and indicating to revert when the performance bug in dynd is fixed.