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/is/is.wren

@@ -1,6 +1,6 @@
 IO.print(Num is Class) // expect: true
 IO.print(true is Bool) // expect: true
-IO.print(new Fn { 1 } is Fn) // expect: true
+IO.print(Fn.new { 1 } is Fn) // expect: true
 IO.print(123 is Num) // expect: true
 IO.print(null is Null) // expect: true
 IO.print("s" is String) // expect: true
@@ -8,7 +8,7 @@ IO.print("s" is String) // expect: true
 IO.print(Num is Bool) // expect: false
 IO.print(null is Class) // expect: false
 IO.print(true is Fn) // expect: false
-IO.print(new Fn { 1 } is Num) // expect: false
+IO.print(Fn.new { 1 } is Num) // expect: false
 IO.print("s" is Null) // expect: false
 IO.print(123 is String) // expect: false
 
@@ -16,7 +16,7 @@ IO.print(123 is String) // expect: false
 IO.print(Num is Object) // expect: true
 IO.print(null is Object) // expect: true
 IO.print(true is Object) // expect: true
-IO.print(new Fn { 1 } is Object) // expect: true
+IO.print(Fn.new { 1 } is Object) // expect: true
 IO.print("s" is Object) // expect: true
 IO.print(123 is Object) // expect: true
 
@@ -24,7 +24,7 @@ IO.print(123 is Object) // expect: true
 IO.print(Num is Class) // expect: true
 IO.print(null is Class) // expect: false
 IO.print(true is Class) // expect: false
-IO.print(new Fn { 1 } is Class) // expect: false
+IO.print(Fn.new { 1 } is Class) // expect: false
 IO.print("s" is Class) // expect: false
 IO.print(123 is Class) // expect: false