Simplify 'Codec' abstraction

[chshersh]

Currently, we have the following Codec abstract data type:

data Codec r w c a = Codec
    { codecRead :: r a
    , codecWrite :: c -> w a
    }

But in this library it's used only for TomlCodec:

type TomlCodec a = Codec Env St a a

It could be a good simplification to combine two and have

data Codec in out = Codec
    { codecRead :: TomlEnv out
    , codecWrite :: in -> TomlState out
    }

type TomlCodec a = Codec a a

What do you think? With this approach, we can have a single module Toml.Codec with all general functions, encoding and decoding functions instead of two modules Toml.Monad and Toml.Code