Add `ComplexRotation <: Rotation{2}`

[hyrodium]

We now have two types for 2D rotations:

• RotMatrix{2} stores four real values as a matrix
• Angle2d stores one real value as a rotation angle

However, a 2D rotation can be parametrized with two real values $(c,s)$ like

$$ R = \begin{pmatrix}c & -s \\ s & c\end{pmatrix}. $$

This rotation is the same as a rotation by a complex number $c+is$, and can be implemented like this:

struct ComplexRotation{T} <: Rotation{2,T}
    c::Complex{T}
end

This is sometimes useful when we need to generate a rotation from 2D vectors.
See rotation_between for example.

Rotations.jl/src/rotation_between.jl

Lines 9 to 15 in f00b07d

	function rotation_between(u::StaticVector{2}, v::StaticVector{2})
	c = complex(v[1], v[2]) / complex(u[1], u[2])
	iszero(c) && throw(ArgumentError("Input vectors must be nonzero and finite."))
	isfinite(c) || throw(ArgumentError("Input vectors must be nonzero and finite."))
	theta = Base.angle(c)
	return Angle2d(theta)
	end