We recently added a check for fs.ReadDirFile in IOFS.ReadDir, but forgot to apply a sort to the
result as defined in the spec.
This fixes that and adds a test case for it.
And implement `fs.ReadDirFile` for `BasePathFile`.
There are other `File` implementations that could also benefit from the above, but
this is a start.
The primary motivation behind this is to allow `fs.WalkDir` use the native
implementation whenever possible, as that new function was added in Go 1.16 with
speed as its main selling point.
This commit also adds a benchmark for `fs.WalkDir` that, when compared to the main branch:
```bash
name old time/op new time/op delta
WalkDir-10 369µs ± 1% 196µs ± 3% -46.89% (p=0.029 n=4+4)
name old alloc/op new alloc/op delta
WalkDir-10 85.3kB ± 0% 40.2kB ± 0% -52.87% (p=0.029 n=4+4)
name old allocs/op new allocs/op delta
WalkDir-10 826 ± 0% 584 ± 0% -29.30% (p=0.029 n=4+4)
```
Fixes#365
afero.Fs/File requires mutating methods implementation but they will always return error because fs.FS is read-only.
ReadAt and Seek will return error if underlying fs.File doesn't implement them.
Bjørn Erik Pedersen