Make constructors just methods.

* Eliminate "new" reserved word.
* Allow "this" before a method definition to define a constructor.
* Only create a default constructor for classes that don't define one.

test/language/constructor/named.wren

@@ -0,0 +1,16 @@
+// TODO: Change this.
+
+class Foo {
+  this named() { _field = "named" }
+  this other() { _field = "other" }
+
+  toString { _field }
+}
+
+IO.print(Foo.named()) // expect: named
+IO.print(Foo.other()) // expect: other
+
+// Returns the new instance.
+var foo = Foo.named()
+IO.print(foo is Foo) // expect: true
+IO.print(foo.toString) // expect: named