when i RemoveAll lib directory, it also Removed libs directory

Fix gcsfs RemoveAll

bugfix: RemoveAll

As explained in
#291 (comment), now that
`afero` supports `io/fs.FS`, it requires `go` >= `1.15.10`.

Rather than pinning to `1.15` and occasionally confusing users who are
on < `1.15.10`, I thought simplest to jump straight to `1.16`.

Especially given `1.18` was just released, the upstream `go` team no
longer even supports `1.16`.

The change to `go.sum` was the result of running `go mod tidy`.

Update `go.mod` to reflect that `afero` requires `go` >= `1.15.10`

And also enable the CI build for Windows.

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

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.