Get rid of the special "call" primitive result type.

Instead, Fn.call(...) is a special *method* type that has the same
special sauce. The goal is eventually to get rid of the primitive
result type entirely.

test/core/function/call_missing_arguments.wren

@@ -1,2 +1,2 @@
-var f2 = Fn.new {|a, b| System.print(a, b) }
+var f2 = Fn.new {|a, b| System.print(a + b) }
 f2.call("a") // expect runtime error: Function expects more arguments.