Provide StoreReader::enumerate to simplify creation of secondary indexes

[adamreichold]

For secondary indexes, it is often necessary to read all documents, compute some function on them and associated the result with a document ID.

Currently, this requires something like

let reader = segment.get_store_reader(1)?;

for doc_id in segment.doc_ids_alive() {
    let doc = reader.get(doc_id)?;

    // Use doc and doc_id here ...
}

which can be simplified to

let reader = segment.get_store_reader(1)?;

for res in reader.enumerate() {
    let (doc_id, doc) = res?;

    // Use doc and doc_id here ...
}

using the method proposed here.

(I added a new method instead of modifying StoreReader::iter to make the change backwards compatible, i.e. possible to include in a point release.)