arity for {|a,| } was -2 and should have been 1

We still use arity value for dispatch which led to four changes:

1. anonrest which is after the a variable in first sentence means it is not really a rest so -2 (1+arity)*-1{rest} becomes 1
2. generic yield must always check on positive arity whether it should be spreadable or not
3. specific yields must also check spreadable
4. ACP should not specify single value it is spreadable (since it will have more than one value).

We need to transition to something instead of arityValue in signature. I think an enum would likely simplify code and be more readable as well.