Make method_call benchmark use inheritance.

This also fixes a bug where constructors weren't being bound
correctly, and eliminates some unneeded instructions when
compiling ifs.

I also tweaked the other method_call languages to match Wren's.
Since Wren needs to do a super call to get to the parent _count,
the other languages do now too.

This is nice too because it means we're benchmarking super 
calls.

test/constructor/superclass.wren

@@ -1,19 +1,28 @@
 class A {
   this new(arg) {
     io.write("A.new " + arg)
+    _field = arg
   }
+
+  aField { return _field }
 }
 
 class B is A {
   this otherName(arg1, arg2) super.new(arg2) {
     io.write("B.otherName " + arg1)
+    _field = arg1
   }
+
+  bField { return _field }
 }
 
 class C is B {
   this create super.otherName("one", "two") {
     io.write("C.create")
+    _field = "c"
   }
+
+  cField { return _field }
 }
 
 var c = C.create
@@ -23,3 +32,7 @@ var c = C.create
 io.write(c is A) // expect: true
 io.write(c is B) // expect: true
 io.write(c is C) // expect: true
+
+io.write(c.aField) // expect: two
+io.write(c.bField) // expect: one
+io.write(c.cField) // expect: c