Setup hooks

[sheaf]

This PR contains the implementation of the new Hooks build-type, as described in the Haskell Foundation Tech Proposal.

It builds on several previous PRs. For review, you should only need to look at the Introduce SetupHooks and SetupHooks: add tests commits.

New modules:

• Distribution.Simple.SetupHooks.Internal contains the internal implementation details of the API for the Hooks build type
• Distribution.Simple.SetupHooks.Rule defines fine grained rules as specified in the proposal, for use in pre-build hooks
• Distribution.Simple.SetupHooks.Errors defines error constructors pertaining to the Hooks build type

There are barely any changes to Cabal outside the introduction of these modules. Essentially, the changes consist of defining defaultMainWithSetupHooksArgs, and the introduction of variants build_setupHooks, configure_setupHooks... of existing functions (with e.g. build = build_setupHooks noBuildHooks) which insert the hooked actions at the appropriate times.

New libraries:

• Cabal-hooks is a new package that exports the exposed interface for package authors to write their own SetupHooks.hs modules. For the time being, it mostly re-exports datatypes from Cabal, together with some slightly higher-level functions for use in the API, all through the single module Distribution.Simple.SetupHooks.
  As explained in the HF tech proposal, a possible future goal would be to make this package no longer depend on Cabal, perhaps making Cabal depend on it instead. This would limit the amount of internal details from Cabal we are exposing through the Hooks API, which would have the potential to improve maintainability of user packages relying on the Hooks API.

TODO:

• Rebase this PR once preparatory MRs land.
• Update this PR once the discussion on the tech proposal settles down, with a final design.
• Ensure all documentation is up-to-date relative to the exposed API.
• Add changelog entry.
• Address the remaining SetupHooks TODOs in the code.

--

[andreabedini]

Great work. Let's merge this in and start using it 💪 ☺️

[alt-romes]

Thank you for your reviews @gbaz @andreabedini.

We're finishing the migration of packages in the testing-overlay to this very latest versions of Hooks to ensure there is nothing missing from this final version of the MR. We'll do the merge right after.

[sheaf]

I've been applying a few fixes, in particular to the Binary instances of datatypes involved in pre-build rule. The main point is that we want to be able to treat arguments to rules as opaque blobs of data on the build system side, which means that any such data must always be prefaced by its length (otherwise the build system doesn't know how much data to read). I am planning to get this merged as soon as CI passes now.