replace resolver by typeless operator executer

[nopeslide]

• Not all operators have an input type constraint, let's call them unconstrained (see constant).
• Unconstrained operators can't be resolved, since no type constraint exists to be resolved.
• All operators have an unconstrained operator implementation (current status, no relation to this issue).
• Constrained operators can't make use of this unconstrained operator implementation.
• All resolvers are autogenerated, but the generated ones for unconstrained operators make no sense (maybe stop generating them?).
• Instead of searching inside the set structure for a resolver, we could always return the unconstrained operator executer.
• The unconstrained operator executer 'knows' if it's constrained and can call the resolver to find its type specific implementation or be itself the actual implementation.
• If any custom implementation is needed, one can directly modify the unconstrained operator without touching autogenerated code or other type specific implementations