Hide some built-in methods from library view

- ClassAttributes and MethodAttributes are abstracted out in
ProtoCore.AST.AssociativeAST namespace. Earlier ClassDeclNode, Function
and Constructor nodes were using such classes defined in ProtoFFI
namespace.
- Provided a mechanism to tag built-in methods with HiddenInLibrary
attribute, so that they can be hidden.
- Hide some of the built-in methods as per 070 release requirement.