Fix returning from constructors (#845)

* Fix returning from constructors

 1. Do not allow returning with a value
 2. Return the instance, correctly, even when the user returned explicitly

* revise error message for consistency, revise implementation details a bit, fix extra args to finishBody

* clarify tests a bit

* document constructor return

Co-authored-by: ruby0x1 <ruby0x1@pm.me>

test/language/constructor/cannot_return_value.wren

@@ -0,0 +1,5 @@
+class Foo {
+  construct new() {
+    return 1 // expect error
+  }
+}

test/language/constructor/return_without_value.wren

@@ -0,0 +1,18 @@
+class Baz {
+  construct new() {}
+}
+
+class Bar {
+  construct new() {
+  }
+}
+
+class Foo {
+  construct new() {
+    return
+  }
+}
+System.print(Baz.new()) // expect: instance of Baz
+System.print(Bar.new()) // expect: instance of Bar
+System.print(Foo.new()) // expect: instance of Foo
+System.print(Foo.new() != null) // expect: true