Add cancellation semantics to decoder

[eliotheinrich]

Description

This PR adds a cancellation_source containing shared cancellation state for ensemble decoders, which can be queried via a cancellation_token. This will be used for orchestrating ensembles of decoders under deadlines. Decoders gain an overloaded decode(syndrome, token) call, allowing them to provide an appropriate behavior for polling the token and aborting early, returning a nullopt. Default behavior is to ignore the token and delegate to decode(syndrome).

The trt_decoder inference does not provide an interface for cancellation, but it does pass the cancellation token to the global decoder. The sliding_window decoder does the same.

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Tests

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  the area being modified.

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