Both GHC and Hugs parse 1e12 as a number, but Yhc don't.

For example, test = print 1e12 gives: yhc: -- during when renaming Error: Identifier e12 used at 7:15-7:17 is not defined.

Malcolm has a fix we need to apply. We also need to introduce a test case.

parsing/literal_rat test case introduced, bug fixed using Malcolm's code.