Introduced on tantivy.git @ e25284ba

This changeset is sufficient, however upstream's f71b04acb introduced
a `debug_assert!(self.doc() <= target)` for `SegmentPostings::seek`
that looks overzealous to me.

In release mode all tests pass, but given that lot has changed since
last I looked I'll be double checking the affected functionality
prior to letting this go wild.