Redo transposed() because.
Fix set_rowvec/colvec.

In addition, set_row/colvec3 no longer modify the fourth row/column
elements when assigning the vector to a given column/row.
Add methods, fill out operators for mat4_t.

* Add negated/negate.
* Add sum, add, scale, and scaled for mat4_t arguments.
* Add assignment operators.
* Fix equality operator. Now allows differing value types, though in
  reality you should probably stick to a single value type for your
  project.
* Add inequality operator (piggybacks equality).
Add mat4::scaling
Add alignas(4) for maths structs.
Update documentation. Add mat4 docs.