More complete support for indexNegative, sliceNegative, sliceMixed sequence methods

[pineapplemachine]

These methods allow more optimal implementations, particularly for unbounded sequence types, of index and slice functions. Sequence types which are created from others should all be investigated for support of these methods; at the time of writing only very few such sequences support them. Unbounded sequence types capable of supporting these methods should all be given support for them; currently only CounterSequence and InfiniteRepeatElementSequence have full support for these new operations.

Indexing and slicing is anticipated to be an uncommon enough use case that this task should be considered fairly low priority.

See #158 - PR where these new sequence methods were added