Allow statement bodies for flow control.

This means this works now:

  if (foo) return bar

test/return/after_else.wren

@@ -0,0 +1,3 @@
+IO.write(fn {
+  if (false) "no" else return "ok"
+}.call) // expect: ok

test/return/after_if.wren

@@ -0,0 +1,3 @@
+IO.write(fn {
+  if (true) return "ok"
+}.call) // expect: ok

test/return/after_while.wren

@@ -0,0 +1,3 @@
+IO.write(fn {
+  while (true) return "ok"
+}.call) // expect: ok

test/return/in_function.wren

@@ -0,0 +1 @@
+var a = "ok"; IO.write(a) // expect: ok

test/return/in_method.wren

@@ -0,0 +1,8 @@
+class Foo {
+  method {
+    return "ok"
+    IO.write("bad")
+  }
+}
+
+IO.write((new Foo).method) // expect: ok