better prediction for dependent sequence

[craff]

By adding a charset or parametrized by the expected value in continuation we should be able to use an exact charset for dependent sequence.

This might impact performance a bit for grammar not using dependent sequence, but we could have two variants of the Comb module, with the same interface, selected automatically or not.

This would give the following type for continuation:

type ('a, 'b) cont = Charset.t * ('b env -> 'a -> Charset.t * ('b err -> 'b))

or

type ('a, 'b) cont = Charset.t * ('a -> Charset.t * ('b env -> 'b err -> 'b))

instead of the current

type ('a, 'b) cont = Charset.t * ('b env -> 'a -> 'b err -> 'b)

This way we can get the charset without evaluating the continuation to the end.

[craff]

With scheduler, prediction are not so important... So won't fix.