fromEnv: accept a record type instead of a row type

[nsaunders]

With this change, fromEnv will accept the desired record type instead of a row type. I'm thinking this should simplify client code most of the time. For example:

Before

type ConfigRep =
  ( "FOO" :: String
  , "BAR" :: Int
  )

type Config = { | ConfigRep }

config :: Object String -> Either (List EnvError) Config
config = fromEnv (Proxy :: _ ConfigRep)

After

type Config =
  { "FOO" :: String
  , "BAR" :: Int
  }

config :: Object String -> Either (List EnvError) Config
config = fromEnv (Proxy :: _ Config)