LEARNER-1696

By transparently proxying between the XModuleDescriptor and the XModule,
and between their runtimes, we can make them act as a single class, so
that we can swap in an actual XBlock instead.

This makes the LMS use an XBlock's student_view, rather than an
XModule's get_html to render for display. However, it does not yet use
wrap_child to handle instructor debug information or url rewriting.

[LMS-219]