Turn on -XMonadFailDesugaring globally.

In the past few years, GHC has been moving to remove the `fail` method
from the definition of `Monad`, a longtime wart, and requiring monads
that fail due to an incomplete pattern match to implement `MonadFail`.
You can read about it[here](https://wiki.haskell.org/MonadFail_Proposal).
Though this move is still in progress, we can opt into it by turning
on the `-XMonadFailDesugaring` extension.

The matching and rewriting systems will both benefit from this, as the
incomplete pattern match in following rewrite rule will crash without
`-XMonadFailDesugaring`, even though the sensible and correct thing
for the rule to do is call out to its `MonadFail` instance:

```haskell
-- crashes the program without -XMonadFailDesugaring
getReceiver :: Rule a (Ruby.Send term) term
getReceiver = do
  (Ruby.Send (Just rec) _ _ _) <- target
  pure rec
```

In addition, turning on `MonadFailDesugaring` will warn you if you put
an incomplete pattern match in a monad that doesn't implement `MonadFail`.
This setting will become implicit in GHC 8.6, so this is a good chance
to make sure that we don't introduce any incomplete patterns going forward.

semantic.cabal

@@ -286,6 +286,7 @@ library
                      , DeriveTraversable
                      , FlexibleContexts
                      , FlexibleInstances
+                     , MonadFailDesugaring
                      , MultiParamTypeClasses
                      , OverloadedStrings
                      , RecordWildCards
@@ -384,6 +385,7 @@ test-suite test
                      , DeriveGeneric
                      , FlexibleContexts
                      , FlexibleInstances
+                     , MonadFailDesugaring
                      , MultiParamTypeClasses
                      , OverloadedStrings
                      , RecordWildCards