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/const.go

@@ -0,0 +1,16 @@
+package fs
+
+import "syscall"
+
+// Flags to OpenFile wrapping those of the underlying system. Not all flags may
+// be implemented on a given system.
+const (
+	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
+	O_WRONLY int = syscall.O_WRONLY // open the file write-only.
+	O_RDWR   int = syscall.O_RDWR   // open the file read-write.
+	O_APPEND int = syscall.O_APPEND // append data to the file when writing.
+	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
+	O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist
+	O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
+	O_TRUNC  int = syscall.O_TRUNC  // if possible, truncate file when opened.
+)