Proposal: System.Environment.getEnv should return Maybe

[Simon Hengel]

> So I propose a function: lookupEnv :: String -> IO (Maybe String)

We already have

    System.Posix.Env.getEnv :: String -> IO (Maybe String)

So for consistency I'd advocate for renaming that to lookupEnv, too.
(keeping getEnv as a deprecated alias, for backward compatibility).

Other than that, +1.

Cheers,
Simon