Add sort()

Return a stream with all values sorted.

Should accept a comparator function and do a lexicographic sort by default.

The issue does not name an implementation file or test. Start by locating the stream transformation API and existing ordering-related tests, then check how other operators handle buffering and callbacks. Done means a sort() stream operation accepts an optional comparator and defaults to lexicographic ordering.

Written by the indexing model from the issue text.