[jextract/jni] Add support for importing nested types

[madsodgaard]

Adds support for importing nested types such as below. They are imported as nested static classes on the Java side.

public class A {                                                                                                                           
  public init() {}                                                                                                                         
                                                                                                                                           
  public class B {                                                                                                                         
    public init() {}                                                                                                                       
                                                                                                                                           
    public struct C {                                                                                                                      
      public init() {}                                                                                                                     
                                                                                                                                           
      public func g(a: A, b: B, bbc: BB.C) {}                                                                                              
    }                                                                                                                                      
  }                                                                                                                                        
                                                                                                                                           
  public class BB {                                                                                                                        
    public init() {}                                                                                                                       
                                                                                                                                           
    public struct C {                                                                                                                      
      public init() {}                                                                                                                     
    }                                                                                                                                      
  }                                                                                                                                        
                                                                                                                                           
  public func f(a: A, b: A.B, c: A.B.C, bb: BB, bbc: BB.C) {}                                                                              
}                                                                                                                                          
                                                                                                                                           
public enum NestedEnum {                                                                                                                   
  case one(OneStruct)                                                                                                                      
                                                                                                                                           
  public struct OneStruct {                                                                                                                
    public init() {}                                                                                                                       
  }                                                                                                                                        
}                                                                                                                                          

[ktoso]

One nitpick then LGTM, I wonder if you could give the FFM version of that a shot? Should be basically the same hmm