Warning: this role is part of the Rakudo implementation, and is not a part of the language specification.
Implements the ability to hold roles to be held for composition.
roughly corresponds to
method add_role(Metamodel::RoleContainer: , Mu )
$role to the list of roles to be composed.
method roles_to_compose(Metamodel::RoleContainer: --> List)
returns a list of roles added with
add_role, which are to be composed at type composition time.