Refactor specification of supported digests

To make the definition of supported digests more clear, we have refactored the
digest package to have a special Algorithm type. This represents the digest's
prefix and we associated various supported hash implementations through
function calls.

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

digest/verifiers.go

@@ -1,8 +1,6 @@
 package digest
 
 import (
-	"crypto/sha256"
-	"crypto/sha512"
 	"hash"
 	"io"
 	"io/ioutil"
@@ -33,7 +31,7 @@ func NewDigestVerifier(d Digest) (Verifier, error) {
 	switch alg {
 	case "sha256", "sha384", "sha512":
 		return hashVerifier{
-			hash:   newHash(alg),
+			hash:   alg.Hash(),
 			digest: d,
 		}, nil
 	default:
@@ -95,19 +93,6 @@ func (lv *lengthVerifier) Verified() bool {
 	return lv.expected == lv.len
 }
 
-func newHash(name string) hash.Hash {
-	switch name {
-	case "sha256":
-		return sha256.New()
-	case "sha384":
-		return sha512.New384()
-	case "sha512":
-		return sha512.New()
-	default:
-		panic("unsupport algorithm: " + name)
-	}
-}
-
 type hashVerifier struct {
 	digest Digest
 	hash   hash.Hash