Implement alternative dense internal representation for CombinatorialFreeModule

[nthiery]

Recall that the (currently misnamed) CombinatorialFreeModule implements free modules with basis indexed by any set of objects. The current internal representation is sparse. In small finite dimension, it would often be preferable to use a dense internal representation, for example, using a FreeModule element.

A typical use case are root in systems, which are perused in the implementation of Coxeter/Weyl groups, Weyl character rings, etc. We can expect that this ticket would yield a major speedup for all those applications.

Mathieu and Sean have started to work on a design document, and hope to work further on this in Fall 2012.

CC: @sagetrac-sage-combinat @saliola

Component: linear algebra

Keywords: days38, free module, CombinatorialFreeModule

Author: Mathieu Guay-Paquet, Sean Carrell

Issue created by migration from https://trac.sagemath.org/ticket/13401