Add convenience method for converting a Range to a start and end index

[lopopolo]

Pretty nice refactorings. I've got a couple of questions around the sys::mrb_range_beg_len and a couple of tests for byteslice that could be added.

I'm wondering whether something like this would ever make sense:

impl for Range {
    pub fn array_accessor(interp: &mut Artichoke, cap: Option<Value>) -> (usize, usize) {
        match Range {
            Range::Valid { start, len } => {
                  //do all the checking of start/len against the cap
                  (start, len)
            },
            _ => Err()
        }
    }
}

☝️ Something that does all the conversion to usize, and just returns the elements to pluck out of a list. I don't think there's anything particular special about a range, except it gives you many ways to access elements in an array in this case.

I'm approving since I don't see anything in particular that could/should break.

Originally posted by @b-n in #1958 (review)