Add ActionRegistry replacement for TempRegistry

[jorisdral]

I've reworked the TempRegistry implementation a bit, and I've renamed it to ActionRegistry to better reflect that it is a registry of monadic actions. The ActionRegistry is introduced in a new module so that the TempRegistry can largely remain in use, which makes the diff of this PR smaller. The intent is to replace every use of TempRegistry by ActionRegistry eventually, probably in a few follow-up PRs. For now, there are two places where we do the replacement already: openSession and closeSession. I've also taken the liberty to revisit openSession and closeSession a bit to make them more exception safe

[dcoutts]

So just trying to summarise the changes vs TempRegistry:

• better naming (commit / release of actions rather than temp and frees resources)
• no support for sharing a registry between threads, but hence also cheaper (IORefs rather than MVars)
• generally cheaper: simple LIFO lists rather than maps

We need SPECIALISE pragmas everywhere in the ActionRegistry.hs.

It looks like ActionRegistry would be a good place to insert tests for exception safety. We could use fs-sim style exception testing to throw exceptions at random points within a transaction (when actions are registered) and verify that everything gets cleaned up properly.

[jorisdral]

So just trying to summarise the changes vs TempRegistry:

* better naming (commit / release of actions rather than temp and frees resources)

* no support for sharing a registry between threads, but hence also cheaper (IORefs rather than MVars)

* generally cheaper: simple LIFO lists rather than maps

Yes, and other changes I can think of:

• runActions tries to run actions even if some of them fail with an exception

• More coherent documentation (I hope)

We need SPECIALISE pragmas everywhere in the ActionRegistry.hs.

Yes, good point. Added them.

It looks like ActionRegistry would be a good place to insert tests for exception safety. We could use fs-sim style exception testing to throw exceptions at random points within a transaction (when actions are registered) and verify that everything gets cleaned up properly.

I have some uncommitted tests for this using IOSimPOR, but it's tricky to get right and I just need to invest more time into it. Using fs-sim to throw errors also works to some extent, but you don't get as precise an interleaving of operations as in IOSim/ IOSimPOR

[dcoutts]

LGTM. Squash fixups and merge I say.