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/readfill_test.go

@@ -0,0 +1,42 @@
+package readers
+
+import (
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type byteReader struct {
+	c byte
+}
+
+func (br *byteReader) Read(p []byte) (n int, err error) {
+	if br.c == 0 {
+		err = io.EOF
+	} else if len(p) >= 1 {
+		p[0] = br.c
+		n = 1
+		br.c--
+	}
+	return
+}
+
+func TestReadFill(t *testing.T) {
+	buf := []byte{9, 9, 9, 9, 9}
+
+	n, err := ReadFill(&byteReader{0}, buf)
+	assert.Equal(t, io.EOF, err)
+	assert.Equal(t, 0, n)
+	assert.Equal(t, []byte{9, 9, 9, 9, 9}, buf)
+
+	n, err = ReadFill(&byteReader{3}, buf)
+	assert.Equal(t, io.EOF, err)
+	assert.Equal(t, 3, n)
+	assert.Equal(t, []byte{3, 2, 1, 9, 9}, buf)
+
+	n, err = ReadFill(&byteReader{8}, buf)
+	assert.Equal(t, nil, err)
+	assert.Equal(t, 5, n)
+	assert.Equal(t, []byte{8, 7, 6, 5, 4}, buf)
+}