Marked the constructor as explicit, because we don't want mistakes like
this to happen:
Matrix4::rotation(1.0f, deg(3.0f)); // oops, swapped axis and angle!
Instead, when calling such constructor, the type must be said
explicitly (initializer-list is forbidden, too):
Matrix4::rotation(deg(3.0f), Vector3(1.0f)); // okay
It should improve performance, because what can be computed at compile
time is now computed at compile time. In addition these things have been
changed or improved:
* Removed constructors from T*, as they cannot be AFAIK written using
constexpr functions only and they only do unnecessary data copying
most of the time. The functionality is now provided using static
functions Matrix*::from() and Vector*::from() which returns either
const or non-const reference to original data, so no copy is
performed. These functions also have explicit warning about unsafe
operations.
* Defaulted copy constructors and assignment operators, using
"default initialization" for arrays instead of memsetting it with
zeros. It should behave the same and this way we don't need any
memcpy(), memset() etc. from <cstring>, which is good.
Multiplication, division, addition and substraction is now done
primarily in assigning operators, as they do the computations in-place,
so they are more memory efficient than implementing them using e.g.
`*this = *this+other;`.
Updated unit test only for matrix multiplication, as vector unit tests
test arithmetic asssign operators too.