digest: add FromString helper to Digest

Signed-off-by: Stephen J Day <stephen.day@docker.com>

digest.go

@@ -80,6 +80,11 @@ func FromBytes(p []byte) Digest {
 	return Canonical.FromBytes(p)
 }
 
+// FromString digests the input and returns a Digest.
+func FromString(s string) Digest {
+	return Canonical.FromString(s)
+}
+
 // Validate checks that the contents of d is a valid digest, returning an
 // error if not.
 func (d Digest) Validate() error {

digester.go

@@ -129,6 +129,11 @@ func (a Algorithm) FromBytes(p []byte) Digest {
 	return digester.Digest()
 }
 
+// FromString digests the string input and returns a Digest.
+func (a Algorithm) FromString(s string) Digest {
+	return a.FromBytes([]byte(s))
+}
+
 // TODO(stevvooe): Allow resolution of verifiers using the digest type and
 // this registration system.