Consider exporting parts of demes/ms.py in a public API.

[grahamgower]

For the the msprime->demes converter, I copy/pasted bits out of the demes/ms.py file. So we should try to rearange some of this so that the useful bits can be made part of the (Builder?) API.

In particular, the migrations_from_mm_list() function might be useful to add as a Builder method. It's also easily tested, because it should round-trip with the Graph.migration_matrices() method.