Allow resolving with 'subtype' aka named dependencies

[karfau]

In other libraries there is a feature that is known as "named dependencies", that allows to differentiate dependencies not only by type, but also by a String.

I think It is not nessesary to restrict this to a String.
Any argument to Resolver.get(...) after the first one could specify a subtype to differentiate what should be returned.

This would require to change the interface in a way that could even allow any level of nesting.

This would also allow to solve #1 in this way:

resolver.get(Provider, Dependency);

which could just return the currently mapped provider from the mapping for the second argument.