Refactored native trait accessors so they are done entirely in roles.

Refactored native trait accessors so they are done entirely in roles.

The class to do the inlining is generated based on the attribute's accessor &
delegation metaclasses, which means this works even when the attribute changes
those classes.

Added code to check if the attr's slot access can be inlined, and to use a
slower but working code path if it can't (see MX::AttributeTree for an example
of why this might be needed).