I miss isolate

The `isolate` function available in both `binary` and `cereal` provides a very useful abstraction.

With `isolate`:
```haskell
isolate (fromIntegral len) $ case cmd of
MCVersion -> MVersion <$> get
MCAddr -> MAddr <$> get
_ -> fail $ "get: command " ++ show cmd ++ " should not carry a payload"
```

Without `isolate`:
```haskell
ensure (fromIntegral len) >>= \bs ->
let f = case cmd of
MCVersion -> MVersion <$> deserialize
MCAddr -> MAddr <$> deserialize
_ -> fail $ "get: command " ++ show cmd ++ " should not carry a payload"
either fail return (runGetS f bs)
```

Getting out of the monad to get right back into it feels clumsy.