Convert NewBlob() away from using resolvedFutures

Instead, use a backing MemoryStore. This is part of removing Futures.

types/string.go

@@ -1,10 +1,6 @@
 package types
 
-import (
-	"bytes"
-
-	"github.com/attic-labs/noms/ref"
-)
+import "github.com/attic-labs/noms/ref"
 
 type String struct {
 	s   string
@@ -15,10 +11,6 @@ func NewString(s string) String {
 	return String{s, &ref.Ref{}}
 }
 
-func (fs String) Blob() (Blob, error) {
-	return NewBlob(bytes.NewBufferString(fs.s))
-}
-
 func (fs String) String() string {
 	return fs.s
 }