Refactor other binary operations over span types to match __add__.

[frreiss]

PR #98 refactored the binary add operation of all the span types (Span, TokenSpan, SpanArray, TokenSpanArray) into a single function add_spans() (located in text_extensions_for_pandas/array/span_util.py) that handles all combinations of the four types in either argument.

Other binary operations should be refactored into the same structure. Here's a list:

• __lt__()
• __le__()
• __gt__()
• __ge__()
• __eq__()
• overlaps()

Note: The above list may not be complete. Please edit this description to add any missing ones.