Renamed the class CppDeclarations => CppDeclarationGenerator to better reflect
its function and emphasize its statefulness.
Renamed Visit{Type,Method} => Include{Type,Method} to clarify that these methods
include the type or method into the generator state.
Renamed GenerateVisitedTypes => GenerateRemainingTypeDeclarations to clarify
that it outputs *remaining* declarations, i.e. declarations that haven't been
generated yet.
We prefer to have the Outputs directory contain just one file or
namespace per output module, so we'll move all C++-related stuff to a
new CppUtils namespace instead.
