Add a few useful date/time types from time (https://hackage.haskell.org/package/time) , e.g.

• POSIXTime
• Date
  etc.

A checklist for where to add things :

• prim constructors go in here : https://github.com/ocramz/heidi/blob/master/src/Data/Generics/Encode/Internal/Prim.hs#L25
• Heidi instances go here : https://github.com/ocramz/heidi/blob/master/src/Data/Generics/Encode/Internal.hs#L312
• lenses go here : https://github.com/ocramz/heidi/blob/master/src/Heidi/Data/Row/GenericTrie.hs#L244