Clean up type system

[MikhailArkhipov]

Clean interfaces:

• IPythonType, IPythonClass, IPythonModule,....
• Remove obsolete methods.
• Derive implementation appropriately (i.e. function is a type).
• Drop .NET/IronPython specific code.
• Make variables store IPythonType rather than IMember.
• Implement MemberId as function of the BuiltInTypeId so they appear consistent.
• Implement IsBuiltIn as a function of declaring module.

[jakebailey]

Related is #346.