Remove DirectoryFrontend (#20)

* efs: Remove DirectoryFrontend.
* run rustfmt.
* Fix some clippy hints.
* efs: Use map_or where possible.
* ondisk: Use size_of instead of hardcoding sizes.
* ondisk: Document try_into_raw_location PhysicalAddress peculiarities better.
* ondisk: Make ValueOrLocation address mode calculations more high-level.
* efs: Update location_of_source comment.
* efs: Make location_of_source more high-level.
* efs: Simplify EfhBhdsIterator.
* ondisk: Clean up comments in DirectoryAdditionalInfo.

Fixes <#8>.

Author: daym