Add support for negative start index in Slice#[start, count]

[ysbaddaden]

Allows slice[-3, 3] to return the last 3 elements of the slice, for example, similar to how Array#[-start, count] behaves, with the difference that Slice returns exactly count elements, while Array returns up to count elements.

Introduces a couple changes:

• negative start now returns from the end of the slice instead of returning nil/raising IndexError
• negative count now raises ArgumentError instead of returning nil/raising IndexError

I believe the current behavior is buggy (unexpected result, underspecified documentation), but this can be considered a breaking change.

refs #14775

[straight-shoota]

suggestion: Describe this additional behaviour in the API docs.

[ysbaddaden]

Note: I added tests to verify that the subslice is pointing to the same memory as the original slice, and not returning a copy. This is the expected behavior, but we didn't check it.