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/function/arity.wren

@@ -1,5 +1,5 @@
-IO.print(new Fn {}.arity) // expect: 0
-IO.print(new Fn {|a| a}.arity) // expect: 1
-IO.print(new Fn {|a, b| a}.arity) // expect: 2
-IO.print(new Fn {|a, b, c| a}.arity) // expect: 3
-IO.print(new Fn {|a, b, c, d| a}.arity) // expect: 4
+IO.print(Fn.new {}.arity) // expect: 0
+IO.print(Fn.new {|a| a}.arity) // expect: 1
+IO.print(Fn.new {|a, b| a}.arity) // expect: 2
+IO.print(Fn.new {|a, b, c| a}.arity) // expect: 3
+IO.print(Fn.new {|a, b, c, d| a}.arity) // expect: 4