This patch introduces TerminalState and changes LexerTransition::mNextState to
be a Variant<State, TerminalState>. This means that SUCCESS and FAILURE no
longer need to be part of State.
Some things to note:
- This simplifies the handling of Lex()'s return value, which is nice.
- The patch splits Terminate() into TerminateSuccess() and TerminateFailure().
- |const State& aNextState| wouldn't work for the first arg to
LexerTransition's ctor due to errors in Variant construction that I didn't
understand. I had to change it to |State aNextState|.
