* First pass at filtered-path endpoint. It seems to be working, but there are tests missing, and possibly some optimization to handle large key sets.
* Vendor go-cmp.
* Fix incomplete vendoring of go-cmp.
* Improve test coverage. Fix bug whereby access to a subtree named X would expose existence of a the key named X at the same level.
* Add benchmarks, which showed that hasNonDenyCapability would be "expensive" to call for every member of a large folder. Made a couple of minor tweaks so that now it can be done without allocations.
* Comment cleanup.
* Review requested changes: rename some funcs, use routeCommon instead of
querying storage directly.
* Keep the same endpoint for now, but move it from a LIST to a POST and allow multiple paths to be queried in one operation.
* Modify test to pass multiple paths in at once.
* Add endpoint to default policy.
* Move endpoint to /sys/access/filtered-path.
