Make a release with type-level metadata

[kosmikus]

Probably based on the type-level-metadata-simpler branch.

Advantages:

• This would finally solve Type level constructor/record information #8, which is open for a very long time.
• This would allow the release of kosmikus/records-sop which has been requested in Release kosmikus/records-sop#1.
• The unreleased work I'm doing on datatype-generic migrations using generics-sop is also depending on these features.
• The changes should be mostly backwards compatible.
• I'd welcome more feedback on the interface, and more experience with the type-level features (see below).

Disadvantages:

• I'm not completely happy with the new features yet, so they'll most likely change again in the future.
• There is no easy way to add this without dropping ghc-7 support.

Any thoughts? @phadej @nmattia @jkarni ?

[phadej]

@kosmikus can you make a PR from your branch, it would be easier to review and comment

[kosmikus]

@phadej Sure. Although it's not documented very well.

[kosmikus]

Done. #31.

[kosmikus]

PR is merged. I'll try to do the release within the next few days unless anyone objects.

[kosmikus]

Released 0.3.0.0 today.