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/class/lowercase_name_inside_body.wren

@@ -12,4 +12,4 @@ class foo {
 }
 
 foo.callFoo // expect: static foo method
-(new foo).callFoo // expect: instance foo method
+foo.new().callFoo // expect: instance foo method