Move `BigMSF` and its related bits into a `big` submodule, and add a `small` submodule currently containing only the small MSF `MAGIC` constant.
Move `BigMSF` and its related bits into a `big` submodule, and add a `small`
submodule currently containing only the small MSF `MAGIC` constant.

willglynn/pdb,willglynn/pdb