Move DelimitedIter from osm-pbf-iter to protobuf-iter

Should [DelimitedIter](https://github.com/astro/rust-osm-pbf-iter/blob/master/src/delimited.rs) get moved from [project osm-pbf-iter](https://github.com/astro/rust-osm-pbf-iter/blob/master/src/delimited.rs) into protobuf-iter? It seems unrelated to OpenStreetMap, and looks generally useful for protobuf decoding. If so, this could be a good first issue for newcomers.

Start by reading src/delimited.rs in osm-pbf-iter and comparing protobuf-iter’s structure. Determine whether DelimitedIter can move without OpenStreetMap-specific coupling; done means reaching a clear decision and, if accepted, relocating the generally useful protobuf-decoding code.

Written by the indexing model from the issue text.