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/hasher.go

@@ -105,7 +105,7 @@ func (h *hasher) hashChildren(original node, db DatabaseWriter) (node, node, err
 	case *shortNode:
 		// Hash the short node's child, caching the newly hashed subtree
 		collapsed, cached := n.copy(), n.copy()
-		collapsed.Key = compactEncode(n.Key)
+		collapsed.Key = hexToCompact(n.Key)
 		cached.Key = common.CopyBytes(n.Key)
 
 		if _, ok := n.Val.(valueNode); !ok {