feat: implement closure over "this" in methods and fix upvalue closing order

Add support for closing over "this" in function closures defined inside methods by naming the receiver local variable "this" in method compilers. Fix upvalue closing to copy the actual value instead of the stack top, and reorder return handling to close upvalues before storing the result. Add test cases for closure over "this", nested closures, and nested classes.

test/closure/reuse_closure_slot.wren

@@ -14,6 +14,5 @@
   }
 }
 
-// TODO: Closing over this.
-// TODO: Close over fn/method parameter.
 // TODO: Maximum number of closed-over variables (directly and/or indirect).
+// TODO: Shadow variable used in closure.

test/this/closure.wren

@@ -0,0 +1,12 @@
+class Foo {
+  getClosure {
+    return fn {
+      return this.toString
+    }
+  }
+
+  toString { return "Foo" }
+}
+
+var closure = (new Foo).getClosure
+io.write(closure.call) // expect: Foo

test/this/nested_class.wren

@@ -0,0 +1,22 @@
+class Outer {
+  method {
+    io.write(this.toString) // expect: Outer
+
+    fn {
+      io.write(this.toString) // expect: Outer
+
+      class Inner {
+        method {
+          io.write(this.toString) // expect: Inner
+        }
+        toString { return "Inner" }
+      }
+
+      (new Inner).method
+    }.call
+  }
+
+  toString { return "Outer" }
+}
+
+(new Outer).method

test/this/nested_closure.wren

@@ -0,0 +1,16 @@
+class Foo {
+  getClosure {
+    return fn {
+      return fn {
+        return fn {
+          return this.toString
+        }
+      }
+    }
+  }
+
+  toString { return "Foo" }
+}
+
+var closure = (new Foo).getClosure
+io.write(closure.call.call.call) // expect: Foo