Data slice mutation support (#11939)

* Mutations of slices of data should preserve relative indexing as well as cow semantics of slices

* Ensure hashes of ranges are uniform to the expected hash for Data

* Correct a few mistakes in the slice mutation tests

* Update sequence initializations to avoid directly calling mutableCopy which prevents slice offset mismatches

* Avoid invalid index slices in creating mirrors

* Restore the original Data description

* Resetting a slice region should expand the slice to the maximum of the region (not a out of bounds index of the backing buffer)

* Remove stray comment and use a stack buffer for sequence appending

* Return false when allocations fail in _resizeConditionalAllocationBuffer (not yet in use)

* Enumeration of regions of a slice should be limited to the slice range in the case of custom backing (e.g. dispatch_data_t)

* adjust assertion warnings for data indexes that are negative

Author: phausler