angle[T](a, b: GVec2[T]) is wrong

According to the comment on the subject proc, it should return the angle between two 2D vectors. It currently does not.
````nim
import vmath
assert angle(vec2(0.0, 1.0), vec2(1.0, 0.0)).toDegrees() == 135.0 # should be 90
assert angle(vec2(0.0, 1.0), vec2(-1.0, 0.0)).toDegrees() == 45.0 # should be 90
assert angle(vec2(0.0, 1.0), vec2(1.0, 0.0)).toDegrees() == 90.0 # should be 180
````