Improve Encoding mappings for pairs of sequence

[n1t0]

Fix #486

Improve the mappings XX_to_XX methods by adding support for pairs of sequence:

• token_to_XX methods now return a tuple with the sequence index and the expected result for pairs of sequence
• char_to_XX and word_to_XX now accept a new argument sequence_index to specify the target input sequence
• Also add a token_to_sequence method that can tell for any token, the associated input sequence.

[Narsil]

The choice I would really think needs to be changed it the variant return type, it's really messy for users I think. I gave an example of something which I feel would both keep backcompat and be cleaner for users.

set_sequence_id being exposed feels odd to me, I think we should probably add a realistic test where this might be used in production to convey why it's there (because destroying sequence sources being irreversible, we need a good reason to expose that IMO). Otherwise we should remove it until a convincing scenario is presented I feel.