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/implicit_receiver/inherited_methods.wren

@@ -20,4 +20,4 @@ class Bar is Foo {
   }
 }
 
-(new Bar).test
+Bar.new().test

test/language/implicit_receiver/instance_methods.wren

@@ -18,6 +18,6 @@ class Foo {
   }
 }
 
-(new Foo).test
+Foo.new().test
 
 // TODO: Need to decide how these interact with globals.

test/language/implicit_receiver/locals_shadow_getter.wren

@@ -14,4 +14,4 @@ class Foo {
   }
 }
 
-(new Foo).test
+Foo.new().test

test/language/implicit_receiver/locals_shadow_setter.wren

@@ -17,4 +17,4 @@ class Foo {
   }
 }
 
-(new Foo).test
+Foo.new().test

test/language/implicit_receiver/nested_class.wren

@@ -36,7 +36,7 @@ class Outer {
       }
     }
 
-    (new Inner).test
+    Inner.new().test
 
     getter            // expect: outer getter
     setter = "value"  // expect: outer setter
@@ -44,4 +44,4 @@ class Outer {
   }
 }
 
-(new Outer).test
+Outer.new().test