Allow intentionally defining an `id` field

[tysonzero]

I would like to allow defining the following:

User
    id Int64
    ...
    Primary ...

• This should suppress all the existing magic such as generating a UserId type.

• UserId would be an EntityField User Int64.

• The User type would include a userId :: Int64 field.

• All the Key-based stuff would be based on the Primary field.

As I have used persistent more and more, I have noticed that a lot of the pain points are either due to not being able to treat regular fields more like id, or due to not being able to treat id more like a regular field. So I would like to slowly unify id with other fields in a backwards compatible way.

This change in isolation is less useful than it will be after changes like #995 are implemented, but I similarly think that #995 is less useful without this change. So I think it makes sense to implement this first in isolation.

[parsonsmatt]

This should suppress all the existing magic such as generating a UserId type.

So that magic is just type UserId = Key User. We still need to define a data family instance for Key User for the PersistEntity class to continue working. That instance would look like:

instance PersistEntity User where
  newtype Key User = UserKey Int64

.. which is the status-quo.

That's the only real concern I have about this request. Otherwise I'm totally cool with it, and have wanted it myself in the past.

[tysonzero]

So after looking through persistent it does seem like the inference-heavy approach does largely necessitate Key being a data family instead of a type family. So yeah just dropping the type alias seems smart.

We mostly use UserId just like any other field and rarely take advantage of the Key machinery. It just would be nice to free up the UserId so that we can start defining it in an external module that db-unaware clients can reference.

So based on the above I will refine it a bit:

User
    id Int64
    ...

• Would not generate a UserId alias

• UserId would be an EntityField User Int64 instead of EntityField User (Key User)

• The User type would include a userId :: Int64 field instead of omitting it.