Nix utilities

[iand675]

This is a bit of a brainstorm:

I think we have a few issues that need to be fixed upstream (in GHC itself, or cabal, etc.)

• Detecting versions of what we're instrumenting
• Sending contexts through to threads that we didn't create
• Cleaning up thread local state more reliably

I could see us providing some nix utilities in the meantime that update wrapped dependencies to use patched versions of async, expose their Paths_ module, etc. This would give us much deeper integration abilities into the broader ecosystem at the expense of tooling being less useful outside of the nix ecosystem.