Function to convert angles into 2D unit vectors

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=49. Original issue from byor...@gmail.com on September 6, 2011, 04:54:01 PM UTC)

Suggested by Anton Kholomiov <anton.kholomiov@gmail.com>:

> - I'm using it with function e (like complex vector representation e^ix)
>
> e a = (cos $ tau_a, sin $ tau * a)
>
> And we can define rectangle point-chain like this:
>
> [e 0, 4 ^_ e 0.25, e 0.5, 4 ^* e 0.75]
>
> With e you can define
>
> unitX = e 0
> unitY = e 0.25
> unit_X = -1 ^* e 0

I don't think 'e' is the best name since it is not very self-documenting (although it is suggestive of the polar form of complex numbers). Also, it should take any Angle instance rather than being limited to CircleFrac. But the idea is good, and we ought to add such a function to Diagrams.TwoD.Util (well... 'direction' is already in that module, and this new function should go in the same module as direction, since they are dual, but perhaps both should be moved to a new module like Diagrams.TwoD.Vector).

[fryguybob]

(Imported. Original comment by byor...@gmail.com on September 8, 2011, 05:49:08 PM UTC)

However, nothing stops us from having a more descriptive name and ALSO keeping 'e' as a short synonym.