Add group_by implementation, and fix minor errors in template lines.

There still is the problem of virtual template functions, that will have to be
fixed soon.
Use type-alias for data-representation type.
Add filter and fold definitions to K3::Collection.
Add some friendly TODOs to K3::Collection.
Add mutation operations (peek/insert/delete/update) to K3::Collection.
Fix missing hanging brace in L.K3.CG.CPP.