Move "level-du" and "reachdown" to their own packages

[vweevers]

bench/lib/level-du.js

Lines 6 to 15 in 1225047

	// TODO: move to package
	module.exports = function ldu (db, callback) {
	const location = reachdown(db).location
	if (!location) {
	return process.nextTick(callback, null, null)
	}
	du(location, callback)
	}

bench/lib/reachdown.js

Lines 3 to 10 in 1225047

	// TODO: move to package (shared with subleveldown, level-test)
	module.exports = function reachdown (db, type) {
	if (typeof db.down === 'function') return db.down(type)
	if (type && db.type === type) return db
	if (isLooseAbstract(db.db)) return reachdown(db.db, type)
	if (isLooseAbstract(db._db)) return reachdown(db._db, type)
	return type ? null : db
	}

And incorporate Level/subleveldown@fbbf525

[vweevers]

Can't think of another use case for level-du, and there's no complexity that's worth hiding in a module.