Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

lib/readers/counting_reader.go

@@ -0,0 +1,28 @@
+package readers
+
+import "io"
+
+// NewCountingReader returns a CountingReader, which will read from the given
+// reader while keeping track of how many bytes were read.
+func NewCountingReader(in io.Reader) *CountingReader {
+	return &CountingReader{in: in}
+}
+
+// CountingReader holds a reader and a read count of how many bytes were read
+// so far.
+type CountingReader struct {
+	in   io.Reader
+	read uint64
+}
+
+// Read reads from the underlying reader.
+func (cr *CountingReader) Read(b []byte) (int, error) {
+	n, err := cr.in.Read(b)
+	cr.read += uint64(n)
+	return n, err
+}
+
+// BytesRead returns how many bytes were read from the underlying reader so far.
+func (cr *CountingReader) BytesRead() uint64 {
+	return cr.read
+}