feat(CAFS): repurpose CAFS acronym as a filesystem property

add an interface that allows Filesytems to distinguish when they use refer-by-hash
by opting into the CAFS interface. In the future we may want to expand this interface
with values for working with hashes, but for now just distinguishing which Filesystems
are content-addressed is enough.

fs.go

@@ -79,6 +79,14 @@ type PinningFS interface {
 	Unpin(ctx context.Context, key string, recursive bool) error
 }
 
+// CAFS stands for "content-addressed filesystem". Filesystem that implement
+// this interface declare that  all paths to persisted content are reference-by
+// -hash.
+// TODO (b5) - write up a spec test suite for CAFS conformance
+type CAFS interface {
+	IsContentAddressedFilesystem()
+}
+
 // AbsPath adjusts the provided string to a path lib functions can work with
 // because paths for Qri can come from the local filesystem, an http url, or
 // the distributed web, Absolutizing is a little tricky

mem.go

@@ -53,19 +53,25 @@ type MemFS struct {
 	Files   map[string]filer
 }
 
-// compile-time assertion that MemFS satisfies the Filesystem interface
-var _ Filesystem = (*MemFS)(nil)
+var (
+	// compile-time assertion that MemFS satisfies the Filesystem interface
+	_ Filesystem = (*MemFS)(nil)
+	// assert MemFS implements the CAFS interface
+	_ CAFS = (*MemFS)(nil)
+)
 
 // MemFilestoreType uniquely identifies the mem filestore
 const MemFilestoreType = "mem"
 
 // Type distinguishes this filesystem from others by a unique string prefix
-func (m MemFS) Type() string {
+func (m *MemFS) Type() string {
 	return MemFilestoreType
 }
 
+func (m *MemFS) IsContentAddressedFilesystem() {}
+
 // Print converts the store to a string
-func (m MemFS) Print() (string, error) {
+func (m *MemFS) Print() (string, error) {
 	m.filesLk.Lock()
 	defer m.filesLk.Unlock()
 
@@ -90,7 +96,7 @@ func (m MemFS) Print() (string, error) {
 }
 
 // ObjectCount returns the number of content-addressed objects in the store
-func (m MemFS) ObjectCount() (objects int) {
+func (m *MemFS) ObjectCount() (objects int) {
 	for range m.Files {
 		objects++
 	}
@@ -241,15 +247,15 @@ func (m *MemFS) getLocal(key string) (File, error) {
 }
 
 // Has returns whether the store has a File with the key
-func (m MemFS) Has(ctx context.Context, key string) (exists bool, err error) {
+func (m *MemFS) Has(ctx context.Context, key string) (exists bool, err error) {
 	if _, err := m.getLocal(key); err == nil {
 		return true, nil
 	}
 	return false, nil
 }
 
 // Delete removes the file from the store with the key
-func (m MemFS) Delete(ctx context.Context, key string) error {
+func (m *MemFS) Delete(ctx context.Context, key string) error {
 
 	key = strings.TrimPrefix(key, fmt.Sprintf("/%s/", MemFilestoreType))
 	// key may be of the form /mem/QmFoo/file.json but MemFS indexes its maps
@@ -321,15 +327,15 @@ func (m *MemFS) AddConnection(other *MemFS) {
 }
 
 type adder struct {
-	fs   MemFS
+	fs   *MemFS
 	pin  bool
 	out  chan AddedFile
 	root string
 	tree *nd
 }
 
 // NewAdder returns an Adder for the store
-func (m MemFS) NewAdder(ctx context.Context, pin, wrap bool) (Adder, error) {
+func (m *MemFS) NewAdder(ctx context.Context, pin, wrap bool) (Adder, error) {
 	addedOut := make(chan AddedFile, 9)
 	return &adder{
 		fs:   m,
@@ -373,7 +379,7 @@ func (a *adder) AddFile(ctx context.Context, f File) (err error) {
 
 	if f.IsDirectory() {
 		var dir fsDir
-		hash, dir = node.toDir(&a.fs)
+		hash, dir = node.toDir(a.fs)
 		if err != nil {
 			return err
 		}
@@ -413,7 +419,7 @@ func (a *adder) Finalize() (string, error) {
 	log.Debugf("adding root directory")
 	root := NewMemdir("/")
 	node := a.addNode(root)
-	hash, dir := node.toDir(&a.fs)
+	hash, dir := node.toDir(a.fs)
 	a.fs.filesLk.Lock()
 	a.fs.Files[hash] = dir
 	a.fs.filesLk.Unlock()

qipfs/filestore.go

@@ -50,6 +50,7 @@ type Filestore struct {
 var (
 	_ qfs.ReleasingFilesystem = (*Filestore)(nil)
 	_ cafs.Fetcher            = (*Filestore)(nil)
+	_ qfs.CAFS                = (*Filestore)(nil)
 )
 
 // NewFilesystem creates a new local filesystem PathResolver
@@ -150,6 +151,9 @@ func (fst Filestore) Type() string {
 	return FilestoreType
 }
 
+// IsContentAddressedFilesystem declares qipfs is content-addressed
+func (fst Filestore) IsContentAddressedFilesystem() {}
+
 // Done implements the qfs.ReleasingFilesystem interface
 func (fst *Filestore) Done() <-chan struct{} {
 	return fst.doneCh