Disambiguate functions in Impl traits

[coreggon11]

Standards in OB come with Trait, defining the interface of the standard, and TraitImpl defining the default implementation of the standard. It is done this way so users can override the standards how they like. However, if Trait contains fn foo(), also TraitImpl contains fn foo(). Then if a user wants to call contract.foo(), they have to specify from which trait they want to call the function. It would be more user-friendly to rename the functions in the default implementation to fn foo_impl() etc.