Allow user-specific conversion of Megaparsec custom error component

[kquick]

The megaparsec parser allows the user to specify a custom error type (the e in Parsec e s`) which the user may use to provide specific errors.

The diagnose megaparsec-compat support simply asks megaparsec for a pretty version of any error generated; if the error is a single line, it's reported with This, if it's two lines, it's This then Where, and if it's more than two lines, diagnose drops the megaparsec error output and simply reports <<Unknown error>>.

In my target situation, I wanted to use Diagnose as the custom error type as well (Parsec (Diagnose Text) Text), but since Diagnose errors are more than 2 lines, the result was simply <<Unknown error>>.

This patch adds a mkReports to the Hint class (the class could have been renamed to adjust to this new functionality, but I wanted to keep the change set minimal). When there is a megaparsec FancyError (i.e. the user's custom error), it calls this mkReports to allow the user to determine how the report should be generated. The standard report described above is provided as a default.

I also provided default implementations for both Hint methods so that the user does not have supply an instance.

[kquick]

And as an example, for my use of Parser (Diagnose Text) Text, I have:

instance HasHints (Diagnose Text) Text where
  hints = const mempty
  mkReports _ pos = reportsOf