fs: Add interface and FS implementations

This adds two implementations of the new `FS` interface: One for the local
file system (`Local`) and one for a single file read from an
`io.Reader` (`Reader`).

internal/fs/stat_windows.go

@@ -0,0 +1,31 @@
+// +build windows
+
+package fs
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+	"time"
+)
+
+// extendedStat extracts info into an ExtendedFileInfo for Windows.
+func extendedStat(fi os.FileInfo) ExtendedFileInfo {
+	s, ok := fi.Sys().(*syscall.Win32FileAttributeData)
+	if !ok {
+		panic(fmt.Sprintf("conversion to syscall.Win32FileAttributeData failed, type is %T", fi.Sys()))
+	}
+
+	extFI := ExtendedFileInfo{
+		FileInfo: fi,
+		Size:     int64(s.FileSizeLow) + int64(s.FileSizeHigh)<<32,
+	}
+
+	atime := syscall.NsecToTimespec(s.LastAccessTime.Nanoseconds())
+	extFI.AccessTime = time.Unix(atime.Unix())
+
+	mtime := syscall.NsecToTimespec(s.LastWriteTime.Nanoseconds())
+	extFI.ModTime = time.Unix(mtime.Unix())
+
+	return extFI
+}