driver: Convert runPipeline to use a free monad

This patch converts the runPipeline function to be implemented in terms
of a free monad rather than the previous CompPipeline.

The advantages of this are three-fold:

1. Different parts of the pipeline can return different results, the
limits of runPipeline were being pushed already by !5555, this opens up
futher fine-grainedism of the pipeline.
2. The same mechanism can be extended to build-plan at the module level
so the whole build plan can be expressed in terms of one computation
which can then be treated uniformly.
3. The pipeline monad can now be interpreted in different ways, for
example, you may want to interpret the `TPhase` action into the monad
for your own build system (such as shake). That bit will probably
require a bit more work, but this is a step in the right directin.

There are a few more modules containing useful functions for interacting
with the pipelines.

* GHC.Driver.Pipeline: Functions for building pipelines at a high-level
* GHC.Driver.Pipeline.Execute: Functions for providing the default
interpretation of TPhase, in terms of normal IO.
* GHC.Driver.Pipeline.Phases: The home for TPhase, the typed phase data
type which dictates what the phases are.
* GHC.Driver.Pipeline.Monad: Definitions to do with the TPipelineClass
and MonadUse class.

Hooks consumers may notice the type of the `phaseHook` has got
slightly more restrictive, you can now no longer control the
continuation of the pipeline by returning the next phase to execute but
only override individual phases. If this is a problem then please open
an issue and we will work out a solution.

-------------------------
Metric Decrease:
    T4029
-------------------------

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/GHC/Driver/Main.hs b/compiler/GHC/Driver/Main.hs
index c3c7cd9e31..3d55e77191 100644
--- a/compiler/GHC/Driver/Main.hs
+++ b/compiler/GHC/Driver/Main.hs
@@ -1564,7 +1564,7 @@ hscSimpleIface' tc_result summary mb_old_iface = do
 -- | Compile to hard-code.
 hscGenHardCode :: HscEnv -> CgGuts -> ModLocation -> FilePath
                -> IO (FilePath, Maybe FilePath, [(ForeignSrcLang, FilePath)], CgInfos)
-               -- ^ @Just f@ <=> _stub.c is f
+                -- ^ @Just f@ <=> _stub.c is f
 hscGenHardCode hsc_env cgguts location output_filename = do
         let CgGuts{ -- This is the last use of the ModGuts in a compilation.
                     -- From now on, we just use the bits we need.