Make IOFS.ReadDir check for fs.ReadDirFile

[bep]

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:

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