Make modes a data kind

[jorisdral]

#10 (comment) suggested documenting what the LMDB Modes are that can be associated with transactions and environments. This PR documents modes, but also proposes a change to their implementation, representing them as DataKinds instead of empty types. They change from ...

data ReadWrite
data ReadOnly

... to ...

data Mode = ReadWrite | ReadOnly

This has some advantages and disadvantages:

• Advantage: We can now restrict mode type parameters to be of kind Mode, preventing arbitrary types from being used as parameters.

  type Environment :: Mode -> Type
  type Transaction :: Mode -> Type -> Type

  Before this change, ReadWrite :: Type and ReadOnly :: Type, so we could have written Environment Word64.
• Advantage: For the same reason, we can remove some constraints that have become redundant (the Mode/IsMode constraint).
• Disadvantage: Downstream code has to enable DataKinds.
• Disadvantage: Downstream code has to disable unticked promoted constructor warnings, or use ticked promoted constructors.

[jasagredo]

This is fine for me.

Downstream code has to disable unticked promoted constructor warnings

This is fine (see)

Downstream code has to enable DataKinds.

This is fine. We don't want any other things to be used instead of those two types.