Overview of required changes to support `SpanGroup` rather than
`Tuple[Span]`:
* implement slice for `SpanGroup`
* return `SpanGroup` for `SpanGroup + x` or `x + SpanGroup` rather than
refusing to concatenate (currently without good error handling)
Side effects:
* if appending to `Doc.ents`, only `Iterable[Span]` is supported rather
than other formats like raw entity tuples from matcher results, but you
can still assign mixed data in any of the currently supported formats to
`Doc.ents`
* for the `Matcher` case, `as_spans` provides a good alternative
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