Some changes to the Vec class and VecTraits in anticipation of creating
Vec-like objects. The following changes are made:

* Add GetNumberOfComponents to Vec, which returns NUM_COMPONENTS.

* Likewise, all VecTraits have a GetNumberOfComponents method.

* The ToVec method in VecTraits is changed to CopyInto so that it can be
used when the length of the Vec-like is not known. CopyInto is also
added to Vec.

* VecTraits has a typedef named IsSizeStatic which is set to
VecTraitsTagSizeStatic when the number of components is known at compile
time and VecTraitsTagSizeVariable when the number of components is not
known until runtime.