Schema naming and whole swagger spec

[fizruk]

As discussed previously in #10 (comment).

When building the entire Swagger spec we can't know for sure all needed schema definitions for a spec to be complete. Here's an example:

data Color = White | Black deriving (Generic, ToSchema)
data Dog = Dog { nick :: String, color :: Color } deriving (Generic, ToSchema)
data Person = Person { name :: String, dog :: Dog } deriving (Generic, ToSchema)

type PersonAPI
  = "person" :> ReqBody Person :> Post ()

How would we know to add Dog and Color to auto-generated swagger spec?

The solution I think should be for ToSchema to generate also a list of used definitions.