Generic instances

[jkarni]

I want to retrieve the list of types of Placeholders in a typechecked Statement. This would be significantly easier with a Generic instance for most datatypes.

I'm happy to send a PR with instances, though looking at the ag files has me a little confused: I'd have thought it to be just a question of adding deriving <Type>: <classes>, but it seems as though ScalarExpr has more instances than I can find in the source file.

[JakeWheat]

The instances are probably from this line which is using the uuagc syntax:
deriving AllNodes: Show,Eq,Typeable,Data
(hssqlppp/src/Database/HsSqlPpp/Internals/AstInternal.ag:1382)

AllNodes is defined just above this.