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/core/map/key_types.wren

@@ -1,4 +1,4 @@
-var fiber = new Fiber {}
+var fiber = Fiber.new {}
 
 var map = {
   null: "null value",

test/core/map/new.wren

@@ -1,4 +1,4 @@
-var map = new Map
+var map = Map.new()
 
 IO.print(map.count) // expect: 0
 IO.print(map) // expect: {}

test/core/map/to_string.wren

@@ -12,7 +12,7 @@ class Foo {
   toString { "Foo.toString" }
 }
 
-IO.print({1: new Foo}) // expect: {1: Foo.toString}
+IO.print({1: Foo.new()}) // expect: {1: Foo.toString}
 
 // Since iteration order is unspecified, we don't know what order the results
 // will be.