function createIdToUrl( db ) {
	var entries;
	var i;

	idToUrl = {};
	entries = objectEntries( db );
	for ( i = 0; i < entries.length; i++ ) {
		idToUrl[ entries[ i ][ 1 ].id ] = entries[ i ][ 0 ];
	}
}
Creates a mapping from link identifiers to link URLs and stores the mapping in  variable `idToUrl`.

@private
@param {Object} db - link database mapping link URLs to metadata