Refactor Vtr::Array for better "const" safety

Const' declared instances of Vtr::Array do not protect the pointer held
privately by the class properly. In order to force the compiler to
protect this pointer, we removed all non-const accessors from Vtr::Array
(now renamed Vtr::ConstArray) and moved them to a child class (Vtr::Array),
which requires const_cast<> operators internally to allow access.

The change & renaming is then propagated to all internal dependencies.