Consider c++ Mixin-stype for augmented data structures

[cwpearson]

For example:

• base CSR
• CSR + COO mixin
• CSR + out-degree map mixin

[cwpearson]

I think most of our code would need to modify what happens during construction of the object, i.e., it would need to do all the original construction operations, plus some additional ones.

• COO mixin would add to edge src array for each edge
• out-degree map mixin would update map with the completion of each row.

The mixin could not replace the constructor, because then it could only be mixed with the base class. It would be possible for the mixin to defer to the base constructor and then run its own initialization, but that part would have to wait until the entire graph was read in instead of being able to overlap it with I/O.