lib: Export IsEquivalentResult from reg

lib/reg.py

 from typing import Set
 from lib.common import Terminal, Nonterminal, Eps
 from lib.grammars import RegGrammar, CFG
-from lib.dfa import DFA
+from lib.dfa import DFA, IsEquivalentResult
 from lib.nfa import NFA
 from lib.regex import RegEx