Removing of another <*stream> #include leads to more compilation time
saving, now from ~5:12 to ~4:55. Another compilation time improvements
will now be possible only by using Clang's modules, I don't know where
to optimize further (except for getting rid of <sstream> in tests).
Currently moved only non-square functionality from Matrix there. Also
static constant members such as row/column count and size are now
lowercase, as they are variables, not types.