Add encode/decode instances for Identity a

[ajnsit]

What does this pull request do?

Adds trivial encode/decode instances for Identity a. I ran into this when trying to encode a data type that looks like this -

data Entity f = Entity
  { guid :: f Guid
  , <some other fields here>
  }

When the entity has a Guid it can be persisted as Entity Identity. This is a very common pattern.

However in the absence of an encode instance for Identity a, I have to manually encode Entity while unwrapping the guid field, which is a bit ugly.