Implement Method#curry, which simply delegates to Proc#curry.

Also fixes an existing bug in JRuby which would ignore the argument
passed to `Proc#curry()` and would instead always use the Proc's arity.
With that fixed, `Proc#curry` properly works with varargs now.

The fixed regression was introduced in 72703450108175e45fb4b21fb7d07676e001c952