Exception safe resource management

Examples:

import Control.IO.Region (region)
import qualified Control.IO.Region as R

...
  region $ \r -> do
    resource <- R.alloc_ r allocate free
    use resource
    -- resource will be automatically freed here

...
  region $ \r -> do
    (resource, key) <- R.alloc r allocate free
    use resource
    if ...
      then R.free key  -- free it earler
      else use resource

...
  region $ \r1 -> do
    resource <- region $ \r2 -> do
      (resource1, key) <- R.alloc r2 allocate free
      use resource
      resource `R.moveTo` r1  -- transfer ownership to region r1
      return resource
    doSomethingElse resource
    -- resource will be freed here

...
  region $ \r1 -> do
    (r2, r2Key) <- R.alloc r1 R.open R.close  -- region is a resource too
    resource <- R.alloc r2 allocate free
    use resource
    r2Key `R.moveTo` r3  -- move region r2 ownership (and also the resource) to other region