trie: rework and document key encoding

'encode' and 'decode' are meaningless because the code deals with three
encodings. Document the encodings and give a name to each one.

trie/node.go

@@ -139,8 +139,8 @@ func decodeShort(hash, buf, elems []byte, cachegen uint16) (node, error) {
 		return nil, err
 	}
 	flag := nodeFlag{hash: hash, gen: cachegen}
-	key := compactDecode(kbuf)
-	if key[len(key)-1] == 16 {
+	key := compactToHex(kbuf)
+	if hasTerm(key) {
 		// value node
 		val, _, err := rlp.SplitString(rest)
 		if err != nil {