Statement range iteration

[digama0]

• Depends on #59 (inessentially)

The main breaking changes are:

• db.segments() is now db.segments(..) and returns an iterator over segments instead of a vector
• db.statement(bstr) takes a byte string (&[u8]) instead of a regular string (&str). This is more convenient for looking up statements that have been retrieved elsewhere in the API, since these are usually Token or TokenRef which correspond to byte strings.
• db.statements_until(label) has been replaced with db.statements_range(..label).

The main new feature is the addition of db.statements_range() which takes a range expression like label1..label2 or ..label or label.. or (Bound::Excluded(label1), Bound::Unbounded), and yields a double ended iterator. This can be used to find the next/previous statement from a given one, or iterate over statements in a range.

[tirix]

I see you have taken care to apply the SegmentId's special order in SegmentOrder::range(), and that a SegmentIter indeed iterates over slices of the ordered SegmentId's, so the obvious pitfall I could think of has been taken care of. I also see how both SegmentIter and StatementIter implement a double ended iterator and can be used to go back along statement addresses, while taking care of the special segment ordering. And I see how both are linked by flat_map in the statement_range_address API.

So this looks good to me! Sorry I have taken time to review this one.