Reduce duplication of code for loading C++ modules.

This also creates the misc sub-module. This may be re-organized in the
future.

Also, get rid of g.underlying_graph(), and rely on "private" variable
g._Graph__graph instead.

The whole histogram code has been redone, and the code has been
simplified. The three-point vertex-edge-vertex correlation has been
scrapped, since it's not frequently used, and would make compilation
even more expensive.

This also adds some missing files to the generation routine.