Make mem.File implement fs.ReadDirFile

Fix sorting in IOFS.ReadDir

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.

Update README.md

go mod tidy

Merge pull request spf13#334 from KastenMike/add-modTime-to-folders

add modTime on folder creation

Merge pull request spf13#332 from pojntfx/master

Call `OpenFile` instead of `Open` in `CacheOnReadFs` (fixes spf13#193)

Merge pull request spf13#333 from drakkan/gcsfs

GCS fs: move all gcsfs related implementations to its own package

Merge pull request spf13#331 from kargakis/gcs

Add GCS Fs implementation

Merge pull request spf13#287 from tdakkota/fix/issue-286

Fix incorrect (*mem.File).WriteAt

Merge pull request spf13#280 from mfuterko/master

Fix a typo