acquire a resource and you use the handle to
release the resource.
acquire and Handle.release are the low-level interface to accessing the resource.
These should be used carefully: to make sure you do not resource leak you need to
call Handle.release at least once on every handle returned by acquire. To ensure
safety you also must stop using the 'a after having called release on a
corresponding handle.
create is the low-level way of creating resources.
Think of create and acquire as inverses: create wraps the resource
implementation and acquire exposes it:
1. acquire (create f) = f ()
2. create (fun () -> acquire r) = r