Extend CharSpanArray and TokenSpanArray to support multiple documents

[frreiss]

The current implementation of CharSpanArray and TokenSpanArray only allows a single target text for all of the spans in a given array. This restriction is fine as long as all the spans in a given Dataframe come from a single document, but it complicates use cases involving combining information from multiple documents in a single Dataframe. Currently the only way to have spans from multiple documents in a series is to convert CharSpanArray/TokenSpanArray arrays into arrays of type Object containing individual CharSpan and TokenSpan objects.

We should extend our span array types to allow for multiple target texts per array. Key challenges to address:

• Memory-efficient representation for the span data
• Clean semantics for span comparison across documents. What happens if two documents in a corpus have the same text?
• Efficient implementations of the span operations under text_extensions_for_pandas.spanner with multiple target texts
• Serialization/deserialization to/from Arrow and Feather format

[frreiss]

WIP PR at #170

[frreiss]

Closed by #170

[frreiss]

Correction: #170 implements everything but updated Arrow support. Opened #179 to cover that work.