* 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)
* Gram-Schmidt using vector type
* QR decomposition
* Operator 1-norm and maximum norm; stub for 2-norm and angle between
subspaces (`matrix-basis-angle')
* `matrix-absolute-error' and `matrix-relative-error'; also predicates
based on them, such as `matrix-identity?'
* Lots of shuffling code about
* Types that can have contracts, and an exhaustive test to make sure
every value exported by `math/matrix' has a contract when used in
untyped code
* Some more tests (still needs some)
