Made Gaussian elimination faster (half the scaled additions) and more
accurate (always produces zeros in the lower half)
Initial `math/matrix' documentation, and some changes

* At least stubbed out all (or almost all) `math/matrix' exports; many
  have complete documentation (e.g. types, predicates, accessors,
  constructors, for loops, conversion, much of "Comparing Matrices")

* Moved `matrix-zero?' and fixed to use (matrix-error-norm) as its norm

* Added `matrix-basis-cos-angle' (currently a stub; should return smallest
  singular value of a certain matrix multiplication)