Implement closest-match method / constructor finder.

[sjoerdtalsma]

Sometimes more than one method or constructor are applicable for an invocation.
Especially if one or more of the arguments are null, there can be multiple applicable signatures.

This library left this up to the JVM to decide (meaning methods can't be resolved if provided with null arguments, because the type is missing).
If, however, there is only a single possible signature, we can actually call that.