`FloatQuaternion` `from:`-`to:`-vector init, with `axisWhenVectorsAreOpposed:` option
`FloatQuaternion` `from:`-`to:`-vector init, with `axisWhenVectorsAreOpposed:` option

In addition to the existing `init(from:,to:)`, adding a variant that takes a `axisWhenVectorsAreOpposed:` axis vector and returns a 180°/π-radians rotation around that axis when the `fromVector` & `toVector` are opposed.  The `simd_quaternion` initializer doesn't handle this case, instead (according to the docs) _“If `from` and `to` point in opposite directions (to within machine precision), an arbitrary rotation axis is chosen”_.

* Added a `init(from:,to:,axisWhenVectorsAreOpposed:)` init that checks if they're opposed and returns a 180°/π-radians rotation around that axis, else just calls the existing `init(from:,to:)` initializer.  
	▸ Far less efficient than the standard simd constuctor, but easier and safer than checking for this special case in one's own code.  Probably not necessary much of the time, but handy when it is.
capnslipp/IntN,capnslipp/IntN