Use aligned storage inside SparseMatrix

Submitted by Christoph Hertzberg @chhtz

Assigned to Nobody

Link to original bugzilla bug (#1729)
Version: 3.4 (development)

Description

Bug #1728 (closed) exposed some non-deterministic behavior involving sparse reductions.
This could be solved by having SparseMatrix (and SparseVector) store their coefficients aligned.

A big drawback is that this would break ABI compatibility.