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

test/language/class/name_inside_body.wren

@@ -11,4 +11,4 @@ class Foo {
 }
 
 Foo.sayName // expect: Foo!
-(new Foo).sayName // expect: Foo!
+Foo.new().sayName // expect: Foo!

test/language/class/new.wren

@@ -1,6 +1,6 @@
 class Foo {}
 
-var foo = new Foo
+var foo = Foo.new()
 IO.print(foo is Foo) // expect: true
 
 // TODO: Test precedence and grammar of what follows "new".