Lazily instantiate TerminalParseFailure.  Saves 25ms avg.
Lazily instantiate TerminalParseFailure.  Saves 25ms avg.

Speedup:
  a = 0.0257397
  b = 0.0244671

xiaohui-zhangxh/treetop,maddenp/treetop,cjheath/treetop,nathansobo/treetop