These changes are basically to support some upcoming changes to
ArrayHandleZip. The major additions are an implementation of VecTraits
for Pair and an overloaded << operator to ostream for Pair.

I also had to declare the operator<< for Pair to be in Types.h. Under
some circumstances the operator has to either be declared before the
template is declared or declared in the vtkm namespace. (The reasons are
described at <http://clang.llvm.org/compatibility.html#dep_lookup> but I
still don't understand.) I tried adding it to the vtkm namespace, but
that caused several of the other operator<< to fail. Since there is no
way to guarantee that Pair.h is declared before, say, ArrayHandle.h, I
moved the implementation to Types.h.

Since I was moving operator<< to Types.h, I went ahead and moved the
TypeTraits and VecTraits to their respective headers. Since Pair is
declared (but not implemented) in Types.h, these templated classes can
be implemented without including Pair.h.