Add options to select GHCi prompt type errors

This SO post implicitly suggests some extra options for reporting type errors for expressions typed into GHCi. When GHCi reports a type error for an expression e typed at the prompt, it has effectively failed to check void e :: IO () and print e :: IO (). It always reports the error for the latter. There are three things a user may want, and I think options should control which ones are shown:

1. Why didn't void e :: IO () check?
2. Why didn't print e :: IO () check?
3. If any type could be inferred for e, what was it?

changed milestone to %8.4.1

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I don't like the idea of adding options here since they will only benefit those who know about them; and I don't think those will be the same people as the people who have trouble with the existing behavior. (And if you really want to know why void e :: IO () didn't type check then you can enter void e :: IO () into ghci; and that's much more obvious than turning on an option.)

I do think that it'd be reasonable to improve the (default) behavior when e type checks but print e does not (presumably due to a missing Show instance).

These tasks won't happen for 8.4 and have been sitting at low priority for quite some time. Consequently I am un-milestoning them. Don't let this stop you, the interested reader, from picking one up, however.

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