Extract generic method prototypes

These feature substituted types according to their declaring generic specialisation, with wildcards that reach top-level being converted to their upper or lower bound depending on usage context.

This commit also includes an incidental fix such that constructors declare their return-type as unit, consistent with the Java extractor.

java/ql/test/kotlin/library-tests/generic-instance-methods/Test.java

@@ -0,0 +1,33 @@
+class Generic2<T> { 
+
+  public Generic2(T init) { stored = init; }
+
+  private T stored;
+
+  T identity2(T param) { return identity(param); }
+  T identity(T param) { return param; }
+  T getter() { return stored; }
+  void setter(T param) { stored = param; }
+
+}
+
+public class Test {
+
+  public static void user() {
+
+    Generic2<String> invariant = new Generic2<String>("hello world");
+    invariant.identity("hello world");
+    invariant.identity2("hello world");
+
+    Generic2<? extends String> projectedOut = invariant;
+    projectedOut.getter();
+
+    Generic2<? super String> projectedIn = invariant;
+    projectedIn.setter("hi planet");
+    projectedIn.getter();
+
+  }
+
+}
+
+