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_unix.go

@@ -0,0 +1,36 @@
+// +build !windows,!darwin,!freebsd
+
+package fs
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+	"time"
+)
+
+// extendedStat extracts info into an ExtendedFileInfo for unix based operating systems.
+func extendedStat(fi os.FileInfo) ExtendedFileInfo {
+	s, ok := fi.Sys().(*syscall.Stat_t)
+	if !ok {
+		panic(fmt.Sprintf("conversion to syscall.Stat_t failed, type is %T", fi.Sys()))
+	}
+
+	extFI := ExtendedFileInfo{
+		FileInfo:  fi,
+		DeviceID:  uint64(s.Dev),
+		Inode:     s.Ino,
+		Links:     uint64(s.Nlink),
+		UID:       s.Uid,
+		GID:       s.Gid,
+		Device:    uint64(s.Rdev),
+		BlockSize: int64(s.Blksize),
+		Blocks:    s.Blocks,
+		Size:      s.Size,
+
+		AccessTime: time.Unix(s.Atim.Unix()),
+		ModTime:    time.Unix(s.Mtim.Unix()),
+	}
+
+	return extFI
+}