Support deriving SafeCopy using Generic about safecopy

[ddssff]

I have done this, the result is here: seereason@0340f83

I expect to start testing it soon. It replaces the existing default signatures of getCopy and putCopy because they are dangerous. - they simply call the serialization method, no migrations are performed, and any embedded safecopy types are not decoded correctly.

One issue is that the order in which the values are output is different, and I don't really understand the order that the current code is using. I will file an issue about this pointing to the test code included in this patch.

[ddssff]

Ok, I understand it better and I've finished the implementation so it inter operates with the current template haskell instances. Now to take it for a spin.

[ddssff]

Still needs some tweaks.

[ddssff]

Release notes:

1. It should now be possible to replace any use of $(deriveSafeCopy 3 extension ''Foo) with a standalone instance such as deriving instance SafeCopy Foo where version = 3; kind = extension. If version happens to be 1 and kind happens to be base you can just add SafeCopy to the deriving clause of the type.

2. hypothesize (but I haven't yet experienced the situation) where there may be places in existing repositories where the old default putCopy and getCopy functions output something that would not be readable by this new instance. As I say, this hasn't happened to me yet, but if it did it might be fixed with an instance such as

deriving instance SafeCopy Foo where version = 3; kind = extension; getCopy = get; putCopy = put

3. The new instances currently require the type parameters to have a Typeable instance. Welcoming suggestions how to avoid this.

4. If you see the following it means you have to derive a Generic instance:
   • No instance for (safecopy-0.9.5:Data.SafeCopy.SafeCopy.GGetCopy
   (GHC.Generics.Rep (ReportMap_4 report))
   safecopy-0.9.5:Data.SafeCopy.SafeCopy.DatatypeInfo)
   arising from a use of ‘safecopy-0.9.5:Data.SafeCopy.SafeCopy.$dmgetCopy’