refactor: split a testing library

[brprice]

We split hedgehog generators, and some tasty glue into its own library
(as part of the Primer package). We split these out to enable reuse of
generators between primer and primer-service in the future. Note that
whilst internal libraries are supported with cabal-version: 2.4, the
ability to set visibility is only available from 3.0.

[brprice]

Notes for reviewers:

We have discussed doing something like this a while ago. I am now doing it so I can use generators in primer-service to test openapi descriptions vs json serialisations (I'll open a new pr for that shortly).

This PR is only moving code around, and I suspect is more bikeshed-able than the actual testing PR -- I am happy to have suggestions on better module/file organisation etc!

[brprice]

@dhess I don't think any nix changes are needed as haskell.nix should take care of everything, but could you confirm?

[dhess]

@dhess I don't think any nix changes are needed as haskell.nix should take care of everything, but could you confirm?

I don't see anything in here that would require Nix changes, no.

[dhess]

I've been wanting this for awhile, thanks!

[brprice]

This is rebased and ready to go, except:

Locally, when building cabal clean ; cabal build -O0 primer-hedgehog (but not -O1), I am seeing the following in my terminal /nix/store/f0rrfmf8kh5bsiql5bbq7m4h3xgihpnx-binutils-2.38/bin/ld: warning: type and size of dynamic symbol `primerzm0zi7zi2zi0zminplace_PrimerziApp_defaultLog_closure' are not defined. I do not see this in CI though, or running nix build -v -L .#packages.x86_64-linux.primer:lib:primer-hedgehog. Does anyone have any idea why this happens, or if it is problematic?

[brprice]

/nix/store/f0rrfmf8kh5bsiql5bbq7m4h3xgihpnx-binutils-2.38/bin/ld: warning: type and size of dynamic symbol `primerzm0zi7zi2zi0zminplace_PrimerziApp_defaultLog_closure' are not defined

This seems to be a haskell.nix devShell + cabal issue: I only see it in a haskell.nix devShell and cannot reproduce with a nixos ghc+cabal. The upstream issue seems to be input-output-hk/haskell.nix#662