Report zonefile parsing & signing progress to the caller.

[ximon18]

This allows callers to report progress, which is especially useful on very large zone files.

For example usage see NLnetLabs/dnst#35.

I'm not very happy with the inplace::Zonefile progress support, but due to the replacement of self via the Bytes split() operation, reset of the start variable and the iterator being mutably consumed by the caller, using a new position tracking variable and returning it via Entry seemed like one easy way of getting the progress information out to the iterating caller code.

Perhaps something like Iterator::enumerate() could be used instead to return the progress offset to the iteration loop?

If we keep the extra var returned with Entry it should probably be made into named struct fields instead of anonymous tuple fields, to be more self-evident what the usize value is. Also, the extra tuple field infects all call sites which is annoying as most don't care about it.

Note: This PR lacks progress notification for NSEC hashing, that should also be added.

Thoughts:

• Progress reporting should be optional, so perhaps the callback should be Option.
• Progress reporting has a cost, perhaps it should be emitted only every N units instead of for every unit change?