Add matrix transpose multiplication

[dataphract]

In collision detection, it's often useful to perform computations in the local space of one of the involved objects.

For instance, given an OBB:

struct Obb {
    half_extents: Vec3,
    orientation: Vec3,
}

the closest point within the OBB to another point can be found like so:

impl Obb {
    fn closest(&self, other: Vec3) -> Vec3 {
        let inv_orientation = self.orientation.transpose();
        let local_other = inv_orientation * other;
        self.orientation * local_other.clamp(-self.half_extents, self.half_extents)
    }
}

other is converted to the local space of self by premultiplying by the inverse of self.orientation, and since self.orientation is an orthonormal basis, its inverse is equivalent to its transpose. However, it's not necessary to actually construct the transposed matrix -- the multiplication can be performed by dotting the columns of the left-hand side with the columns of the right.

Ideally, we could instead write:

impl Obb {
    fn closest(&self, other: Vec3) -> Vec3 {
        let local_other = self.orientation.mul_transpose(other); // or transpose_mul(), if you like
        self.orientation * local_other.clamp(-self.half_extents, self.half_extents)
    }
}

[bitshifter]

Created #494 for this.