Pointer receiver on Clear

[vdemario]

The coexistence of (s Set) and (s *Set) can cause problems. For instance, if a consumer of the library were to create an interface to represent a subset of methods Clear can't go on the same interface as the other methods since Set and *Set are treated as two different types.

There are a few ways to handle this:

1. The implementation of Clear could be replaced by a slower version that calls Pop until the Set is empty.
2. Clearcould be removed altogether, leaving the consumer to implement a for loop calling Pop if necessary.
3. Alternatively, all other methods could be changed to use *Set. This may seem heavy handed at first but since it's not necessary to explicitly take the pointer of a Set variable to call *Set methods this incurs no penalty on the consumer. If #1 is implemented it would be a nice symmetry to have the mutable type use a pointer and the immutable type avoid it.

[rjfonseca]

I like option two. The set could be cleared simply with s = Make()