The latest Hackage versions now compile with HEAD.

For now, we point the `build-9.4` job to the URL that it happens to be using at
the moment for release bindists. It is possible that this URL may change in the
future, however.

We move around some uses of `coerce` to avoid coercing underneath `IO`, which
triggers ghc/ghc#22519 on HEAD.

Using 'needs' broke CI:
https://gitlab.haskell.org/ghc/head.hackage/-/pipelines/57782

'dependencies' doesn't care if the dependency exists or not, but 'needs'
does.

;)

Run jobs greedily rather than relying on staging.
https://docs.gitlab.com/ee/ci/yaml/#needs

Further improves the life of head.hackage CI dungeoneers.

This expands the number of modes from two to four, allowing one to
separate running the build from running tests.

This change is backwards-compatible. No existing behavior is changed.

Fixes #57.

This includes `proto3-wire`, which now compiles on HEAD by virtue of
`word-compat-0.0.6` exporting compatibility shims for `Word64`/`W64#`.

These are required to satisfy the new quantified superclasses for
`{Eq,Ord,Read,Show}1`.

Patch taken from
https://github.com/kazu-yamamoto/unix-time/pull/59

Currently, `singletons-base-3.1.1` doesn't support GHC 9.2, which complicates
the GHC 9.2 build job. Let's just pin the CI to `singletons-base-3.1` (which
does support 9.2) until we figure out what to do with the 9.2 job.

This needs to be patched for the same reasons that servant-0.19 needs to be
patched. While we're in town, let's update the revisions used in servant.cabal
as well.

This is a simplistic, CPP-based workaround that doesn't change the API of
`ghc-debug-common`. For a more thorough fix that does change the API, see
ghc/ghc-debug!11.

Adapted from https://github.com/bennofs/th-lift-instances/pull/33

The `vector-algorithms` patch is inspired by
https://github.com/erikd/vector-algorithms/pull/40.

cassava-0.5.3.0 supports vector-0.13.*, so it doesn't need a patch.

Agda defines its own `applyWhen` function, so use explicit imports from
`Data.Function` to avoid name clashes.

The latest Hackage version supports HEAD.