Nullable support

[joel-bach]

This PR aims to fix #55 and includes the changes from #73 but extends those to include support for nullable and correct the behavior for deserialization as well.

Example of the newly generated nullable types:

data X = X {
  requiredAndNullable :: (OpenAPI.Common.Nullable GHC.Types.Int),
  optionalAndNullable :: (GHC.Maybe.Maybe (OpenAPI.Common.Nullable Data.Text.Internal.Text)),
  optionalAndNonNullable :: (GHC.Maybe.Maybe Data.Text.Internal.Text)
}