core/state: access trie through Database interface, track errors (#14589)

With this commit, core/state's access to the underlying key/value database is
mediated through an interface. Database errors are tracked in StateDB and
returned by CommitTo or the new Error method.

Motivation for this change: We can remove the light client's duplicated copy of
core/state. The light client now supports node iteration, so tracing and storage
enumeration can work with the light client (not implemented in this commit).

trie/proof.go

@@ -125,7 +125,7 @@ func VerifyProof(rootHash common.Hash, key []byte, proof []rlp.RawValue) (value
 }
 
 func get(tn node, key []byte) ([]byte, node) {
-	for len(key) > 0 {
+	for {
 		switch n := tn.(type) {
 		case *shortNode:
 			if len(key) < len(n.Key) || !bytes.Equal(n.Key, key[:len(n.Key)]) {
@@ -140,9 +140,10 @@ func get(tn node, key []byte) ([]byte, node) {
 			return key, n
 		case nil:
 			return key, nil
+		case valueNode:
+			return nil, n
 		default:
 			panic(fmt.Sprintf("%T: invalid node: %v", tn, tn))
 		}
 	}
-	return nil, tn.(valueNode)
 }