Add "arity" getter to Fn.

test/function/arity.wren

@@ -0,0 +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