`dupIO`

See (Haddock) documentation of dupIO for rationale and discussion.

Test suite

The test suite is written in a very particular way, to try and ensure that it is robust and specific about what happens when:

We run with -fno-full-laziness and -fno-state-hack, ensuring that ghc won't move let bindings out or in, respectively.
We redefine IO a as
```
type IO a = State# RealWorld -> (# State# RealWorld, a #)
```
This allows us to distinguish between an IO action which allocates something when it is called (\w -> let x = ..) versus an IO action which references something that has already been allocated (let x = .. in \w -> ..).
We do not use any abstractions such as >> or >>=, instead writing everything out. See the LeakInIO demo for motivation.
The tests follow the same general structure outlined in the Sharing, Space Leaks, and Conduit and friends blog post, with some action referencing a conduit (i.e., this is of the form let conduit = .. in \w -> ..), and then a top-level exception handler retry that might try an action again after it has executed.
We set up test infrastructure so that we can verify within the test suite whether a particular case has the memory behaviour we expect. In addition, all test cases are marked with "OK" for tests that run in constant space versus "OOM" for tests that do not. We can therefore do a sanity check that the "OK" tests really are OK by running
```
cabal run test-dupIO -- -p OK +RTS -s
```