Remove constructors

[zbynek]

The point of having StringProvider and StreamProvider is that the parser itself should not import InputStream, instead client code can wrap either strings or streams in a Provider and pass it to the parser. That way client code can be easily compiled with GWT by just removing the StreamProvider from the classpath.

In a recent commit this separation was broken by calling StreamProvider constructor from the generated parser -- that should be removed. This PR also removes the call to StringProvider constructor to maintain symmetry (client code that only needs stream-based parsing should be allowed to remove StringProvider from the classpath as well).