Expose seq field for variable arity definitions

In 2.11 we've changed TupleClass, ProductClass and FunctionClass
endpoints to be exposed as (Int => Symbol) functions that never throw
exceptions but rather return NoSymbol instead of previous error-prone
indexed access on array that could explode.

While simplifying one use case (indexed access) it complicated
ability to check if symbol at hand is in fact a tuple, product or
function:
```scala
  (1 to 22).map(TupleClass).toList.contains(symbol)
```
To cover this extra use case we add a seq method to the variable arity
class definitions that exposes a corresponding sequence of class symbols:
```scala
  TupleClass.seq.contains(symbol)
```
review @retronym