[C++ code gen, dictionary] Make instance names more consistent

[bocchino]

When an instance I is defined in a module M, the name of the instance is M_I. However, the code generation is not entirely consistent:

1. In the dictionary, the name becomes I instead of M_I. This makes the name shorter, but it can create ambiguity because it deletes the module prefix. E.g., M_I and N_I have the same name I in the dictionary. We should be consistent and use M_I in this case; a user who wants the unqualified name I can define the instance I outside of any module scope.
2. In the generated C++, the name becomes I inside the namespace M and M_I outside the namespace M. This makes it hard to write initialization code that works in all cases. Instead of M_I we should use a C++ namespace qualifier M::I.

We can fix (1) as we develop the new JSON dictionary. We can fix (2) by updating fpp-to-cpp.