Add support for operators that return values of the type of the type class parameter

[nrinaudo]

For a given type class F[T], Simulacrum can currently generate operators for all methods of the shape def f(t:T): X. It would be nice if it also supported operator generation for methods of the shape def f(): T.

As a concrete example, consider the following type class:

trait Decoder[T] {
  def decode(s: String): T
}

It would be convenient for String to be enriched with a decode operator, enabling code such as "123".decode[Int] (provided a Decoder[Int] is in scope).

A possible issue would be that, in order to support subtyping of type classes, Simulacrum would need to generate 2 classes for each enhanced type - Decoder.StringOps and Decoder.AllStringOps in our example, for instance.

See this gitter thread for additional context.