Let’s see how it goes with a very flat hierarchy and no .h extension.

This implies creating a trimmed down branch that contains almost
nothing. It will then be usable by projects that do not need the
complex build system, and by the engine itself as a submodule from
the same repository.

This allows us to manipulate polynomials of polynomials. Not sure why yet,
but it could be useful to implement 2-variable polynomials.

This will let us detect problems with unused return values. For instance,
if you don’t remember that normalize(q) returns a pointer and expect it
to modify the object, you’ll get this warning:

warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
    quat q(1,2,3,4);  normalize(q);
                      ^~~~~~~~~ ~

The containers no longer force the user to use the ptrdiff_t type for
size information. For convenience, size() now always returns an int,
and the size_s() method offers a way to handle arrays with more than
2 billion elements. Internally, we still use ptrdiff_t, though.

Since so much code had to be changed, I took the opportunity to get
rid of capitalised accessors for classes that are lowercase.

polynomials as arguments so as to compose them together.