fix(xan): fix compat with new base+ghc r/1187

A couple of changes necessary to get things working with both ghc 8.8.3
and the new base:

- Explicitly import fail from Control.Monad.Fail in the prelude, since
  it's there instead of the base prelude now
- GHC no longer allows type family equality constraints in quantified
  constraints - which is a bummer - but is avoidable in the one case
  where it was happening
- Explicitly import a constructor from Data.List.NonEmpty

Change-Id: Ia06fc724ddc2d6a3f9024c047ed55eea40bcf408
Reviewed-on: https://cl.tvl.fyi/c/depot/+/744
Tested-by: BuildkiteCI
Reviewed-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

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