Make Req an instance of MonadUnliftIO

[sjshuck]

It seems that the monad-control package is only used to make instances for MonadBase IO Req and MonadBaseControl IO Req. Since Req is implemented as a newtype wrapping ReaderT _something IO, it's safe to do

instance MonadUnliftIO Req where
    withRunInIO = liftBaseWith

thereby enabling req consumers following the ReaderT pattern to do all the things possible with monad-control without incurring it as a dependency.
Even if the monad-control instances are offered long-term, for backwards compatibility or any other reason, at least please offer MonadUnliftIO in addition. The unliftio-core package is tiny and the implementation is just these two lines.
Thanks!

[mrkkrp]

I don't mind adding the MonadUnliftIO instance in addition to MonadBase IO Req and MonadBaseControl IO Req. It is somewhat unfortunate to have to support two different solutions to essentially the same problem, but I don't see a better option. The monad-control package is older and I don't want to drop support for it just like that—some people may use it.

[mrkkrp]

If you want to add the instance, please open a PR.