Feature Request: DeleteRange with open range

Would be nice if `DeleteRange` added an overload where begin and/or end are optional which means that it's open ended. Similar to lower/upper bound in the iterator API. I guess open begin could be emulated with `Slice("")`. However, open end is a bit trickier...

Start by locating the DeleteRange API and reviewing the iterator API's lower and upper bound behavior. Compare the existing Slice("") approach for an open beginning with the requested open-ended behavior, then define and test overloads for optional begin and end bounds.

Written by the indexing model from the issue text.