The submodule type has been implemented as a C type. When opening
a submodule's repository this leads to the bug that instead of an
actual pygit2.Repository being instantiated we only create an
object of the C Repository type.
As this is not trivially fixed within the C code, reimplement the
submodule type as a Python interface with CFFI. As submodules
provide no functionality that is usually accessed repeatedly the
code paths should not prove performance critical. In addition,
maintainability is improved by this reimplementation.
