Improve the system for handling nClass inheritance

[perrydv]

This PR has several pieces:

• establishes a delayed handling of inheritance relationships (so that an nClass can be defined even before its base class)
• makes an nClass (via its NC_InternalsClass) contain all_methodName_to_cpp_code_name to track the C++ version of method names obtained from base classes.
• updates some of the internal name management, particularly making cpp_code_name not always get a unique suffix.
• allows multiple inheritance from genericInterfaceBaseC via an interface_resolver<>, which in turn allows base classes to be interfaced from R separately from their derived classes.
• makes more proper use of static and dynamic casting of shared_ptr in C++.

The delayed handling of inheritance happens in three steps, from nCompile:

• Use method connect_inherit for each nClass. This lets everyone find their base classes.
• Use method process_inherit for each nClass. This pulls together method names from base classes.
• Use function NC_check_inheritance for each nClass. This checks for (disallowed by default) method overloading and mismatched method signatures (there are nOptions to control these rules).