Avoid dangling symbols

[arthurpaulino]

@porcuquine said;

[...] We should try to work toward a world where the default is for symbols to be interned.

In terms of engineering, this means that State and Package should be the (only?) ones instantiating (interning) symbols from paths.

#1189 points to the possibility that Lang should hold a map from SymbolRef (Arc<Symbol>) instead