Adds Matrix type representing a sparse matrix
Adds Matrix type representing a sparse matrix

This type is used specifically to represent our sparse exact cover
matrix. We could have created something more like the official
container/ring package, but until there is further justification
for such a package, the basic element as described in Knuth's
paper will suffice.

Additional future considerations may include:
1. hiding the links to attemptto enforce continuity more safely
2. creating an interface type to enforce the column, head, element
   distinctions (would likely require 1)

jessecarl/goDLX