Moved Var type stuff to VarType.hpp; renamed detail/vartype.hpp to var.hpp.
Var: added const_iterator begin() and end() stdlib mimics.¹

1: Sigh.
VarTemplate: defaulted cctor and copy operator=().¹

1: Unfortunately, std::initializer_list<T> only provides const member access,
which makes dealing with VarTemplate containers really painful.