## Changes
This enables the use of `io/fs` functions `fs.Glob` and `fs.WalkDir`
with filers.
We can't use `fs.FS` as the standard interface instead of `filer.Filer` because:
1. It was made for reading from filesystems only, not writing
2. It doesn't take a context for the core functions
Therefore a wrapper will do.
## Tests
* Added unit tests to cover the adapter through a fake filer.
* Manually ran `fs.WalkDir` against both WSFS and DBFS filers.
Pieter Noordhuis