Add 'etaReaderT' function

[chshersh]

This function is introduced in the following blog post and helps with optimizing Haskell code:

• https://www.joachim-breitner.de/blog/763-Faster_Winter_5__Eta-Expanding_ReaderT

It has the following defintion:

etaReaderT :: ReaderT r m a -> ReaderT r m a
etaReaderT = ReaderT . oneShot . runReaderT

It can be used if the code like on the snippet below is called in a loop multiple times:

step :: Instr -> ReaderT Config IO Result
step e = case e of
    Add -> do stuff …
    Del -> do stuff …

You just do:

step :: Instr -> ReaderT Config IO Result
step e = etaReaderT $ case e of
    Add -> … do stuff …
    Del -> … do stuff …