recutils – Add support for indexes

by Michał Masłowski for GNU Project

The aim of the project is to make queries over big recfiles faster by using a binary index storing a mapping of values of specified keys to records. The index will be completely optional and will be ignored if the recfile was modified later than specified in the index. The indexed fields will be specified in the record descriptors and the index will be built (initially or after a change to the recfile) offline by user's request.