Vector4 docs

[Chaosus]

Fully documented Vector4 struct,
+few fixes to other things
+tests
+added missing Transform overloads

[nkast]

This PR brought up something strange with how Length() , LengthSquared() and static Normalize(ref Vector4 value, out Vector4 result) are implemented. Since zero is now readonly, you had to replace the DistanceSquared(ref this, ref zero, out result) with the slower DistanceSquared(this, zero).
The use of DistanceSquared(...) was inefficient in the first place. Same problem with Vector3 while Vector2 has those inline.
@tomspilman, @KonajuGames , should we rewrite those in the future?

[Chaosus]

@nkast Thanks, I fixed it. please said me if I incorrect.

[nkast]

Actually I was talking about inlining Length(), LengthSquared() and Normalize(ref Vector4 value, out Vector4 result). My plan was to submit the changes after this PR was merged.
I thing the rule here is to get 'ref' methods optimized and the slower counterparts call the ref methods.
But the inline of Normilize() here is good, this roundtrip of this is too much and one can argue that a member method is not the same as the 'static' method.

[Chaosus]

@nkast Ok. I left these changes for you.
@tomspilman @KonajuGames This is good PR or I should change it a bit ?

[Chaosus]

@tomspilman @KonajuGames So.. what about merging ?

[Chaosus]

@tomspilman Please react

[tomspilman]

We need a unit test for this as there isn't one and we need to be sure this didn't break anything.

[tomspilman]

Or if @nkast plans to re-write this then I can merge as is. He will just need to write the tests for it then.

[Chaosus]

So I made these tests.. it is ok ?

[tomspilman]

Ok... merging!