Add {Enumerator,Range}#length aliases for their respective #size's

`Array`, `Hash`, and `String` support both `#length` and `#size`. `Range` and `Enumerator` recently added `#size` but do not remain consistent with the rest of core with the exclusion of `#length`. This pull request aliases `Range`'s and `Enumerator`'s `#size` to `#length`.