Fix compilation for haskell-src-exts >= 1.16.0

Fixes compilation for haskell-src-exts >= 1.16.0 which made two relevant incompatible changes (per the [changelog](https://hackage.haskell.org/package/haskell-src-exts-1.16.0.1/changelog)):

* GuardedAlt and GuardedAlts types are replaced with the isomorphic GuardedRhs and Rhs types
* PatBind no longer contains the optional Maybe Type field. The type
  signature is now represented as part of the pattern (PatTypeSig)

The first is trivially fixed due to the isomorphism, the second because the field in question wasn't used by the plugins.