Disambiguate index property stored methods correctly

[fourls]

When a property has an index specifier, its stored specifier must be a function with a single Integer argument.

[fourls]

I've also fixed the duplicate processing of the stored expression, but not the others since that's covered by #377.

[cirras]

I've also fixed the duplicate processing of the stored expression, but not the others since that's covered by #377.

This actually failed the StoredProperties test (featuring a stored specifier backed by a constant) because the top-level PropertyNode visit calls disambiguateParameters and erroneously eliminates non-invocable declarations.

I've restored the double-visit by the SymbolTableVisitor.

I'll rebase #377 over these changes and fix the double-visit of stored in the same commit read and write.