Implement Maybe related things for Esqueleto.Record

[parsonsmatt]

This PR addresses #347 #344 and also #271 potentially

This is a bit of a tricky problem. We previously have two "places" to put a Maybe:

SqlExpr (Value (Maybe  a))
SqlExpr (Maybe (Entity a))

However, the esqueleto records pulls the SqlExpr inside of the record. Given a type:

data X = X { x :: Int }

-- we generate:
data SqlX = SqlX { x :: SqlExpr (Value Int) }

One approach is to create a new type.

data MaybeSqlX = MaybeSqlX { x :: SqlExpr (Value (Maybe Int)) }

The approach I take here is to use Maybe directly:

type MaybeSqlX = Maybe SqlX

The "approved" way of accessing into that is to use the HasField instance, which I define via TemplateHaskell for each field on SqlX:

instance HasField "x" (Maybe SqlX) (SqlExpr (Value (Maybe Int))) where
    ...

Except- wait, this doesn't make sense D:

The ToMaybe instance for records is Just, but that's no good. It means that a LEFT JOIN should give us something which we can case on. But this will be the Wrong Thing - we can't case on the SQL value itself.

So maybe need to rework this. But it's in a good enough state for now.

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