Syb show to fay

[chrisdone]

This introduces the following breaking changes to Fay.Convert:

• readFromFay now has a Data constraint instead of Show
• Aeson support has been changed to instead of use explicit parsers for ints/integers/etc, just use Aeson's own parsers. This avoids needing separate CPP #ifdefs for <0.7 and >=0.7.
• Drops support for Rational and Integer (see below for migration steps). The reason is that neither was serialized in a way that would roundtrip for all values. Also, for similar reasons, fromRational is potentially divergent for aeson's new use of the Scientific type.

And adds the following features:

• Better performance due to now using syb instead of parsing the results of show.
• Allows the serialization and deserialization to be customized on a per-type basis, via encodeFay and decodeFay.

To migrate code using Rational or Integer, use encodeFay and pass an argument e.g. something like

(\f x -> maybe (f x) myIntegerToValueConversion (cast x))

and likewise to decodeFay.

[bergmark]

Thanks a lot! I've been wanting to get rid of pretty-show for a long time. The new serialization option is just gravy :-)