Adapt to MonadFail changes in GHC 8.8.

[quasicomputational]

Note that this is an enforced public API change.

I went with changing the MonadFile typeclass rather than threading
through a MonadFail at every use-site, but that would be possible as
well if it makes more semantic sense - however, since MonadFile
already seems to require an IO-ish implementation, adding that
superclass doesn't seem like an imposition.

[Anton-Latukha]

Migration is probably widely known.

Citing release docs:

The fail method of Monad has been removed in favor of the method of the same name in the MonadFail class.

MonadFail(..) is now exported from the Prelude and Control.Monad modules.
The MonadFailDesugaring language extension is now deprecated, as its effects are
always enabled.

Link: https://gitlab.haskell.org/ghc/ghc/wikis/migration/8.8#base-41300
Proposal: https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail

I'm writing error-handling PR, so, looked at code, and 👍

[Anton-Latukha]

Travis timed-out due to your commit got into timing of compiling Cabal & such.

If you want - you can re-push this change, so Travis compiles it.

[jwiegley]

I restarted the builds.

[quasicomputational]

I've given it another kick; let's see if the cache is full enough for it to complete now...

[quasicomputational]

Doesn't seem like the cache is warming up at all. Cabal keeps getting rebuilt each try. I thought cachix would passively upload artifacts for later re-use?

[vaibhavsagar]

I think cachix only uploads after a build succeeds, so we would need to upload a built Cabal out-of-band to get past that issue.

[jwiegley]

Trying to build and upload to cachix now.

[jwiegley]

I'm having all kinds of problems building this branch...

[sjakobi]

Note that this branch doesn't currently build with cabal because regex-tdfa-text still needs a release compatible with GHC-8.8. See hackage-trustees/regex-tdfa-text#3.

EDIT: It now seems that regex-tdfa-text will not be updated, and that instead regex-tdfa will gain support for Text: haskell-hvr/regex-tdfa#4

[sjakobi]

I think it might be a good idea to include these changes in the upcoming release, even if real GHC-8.8 support is still blocked by regex-tdfa[-text].

Users who don't mind depending on this fork could then still easily compile with GHC-8.8. Users of GHC-8.4 and 8.6 don't lose anything.

[jwiegley]

I just need to find time then to reproduce this build and make sure it works for me before uploading.