Support directory partitions

[crclark]

All operations in the directory layer currently assume that directories are not partitions. They need to support partitions and check whether the given directory is a partition when opening it.

We also have to deal with the fact that partitions don't support all of the operations of normal directories. This will most likely require splitting Directory into two types. Other bindings just throw if an unsupported operation is called on a partition; we should try to avoid that by separating directory operations into type classes or something.

[crclark]

Found a TODO comment in the code which I am removing, and immortalizing here instead:

-- TODO: in the other bindings, there is an abstract directory interface
-- and two implementors of that interface. DirSubspace is the standard one,
-- and DirPartition is the weird one that throws exceptions for most of the
-- functions in the directory interface. In the short term, I think we need only
-- support DirSubspace. In Java, DirPartition inherits from DirSubspace.
-- They both contain an internal DirectoryLayer object that does the real work.
-- We should export a sum type where the constructors are
-- DirSubspace and DirPartition. Or, since DirPartition supports only a subset
-- of operations, make DirSubspace and DirPartition separate types implementing
-- different type classes, so we don't have excessive partiality.