GetSlice can now use both `usize` and `Option<usize>`

Created by: stratact

• Along with removing the now deprecated OptionSlice
• Implemented the AsOption trait for converting either usize or Option<usize> to Option<usize> (credit goes to @LazyOxen for writing out the functionality in the Rust Playpen)

So you can have your 🍰 and eat it too.

The only downsides to doing this are:

• That you cannot do a usize..Option<usize> or Option<usize>..usize because it's statically dispatched
  • So it must be either usize..usize or Option<usize>..Option<usize>
• The test!(array.get_slice(..) == &array); test errors, because when using .. ranges, GetSlice needs to know the type parameters explicitly and I didn't feel like with filling it in for now.

But other than that, I Feel Good!