Change the structure of the GenT monad, to better track explanations.

[TimSheard]

The GenT monad is now a reader of not just Mode, but (Mode,[NE.NonEmpty String])

The GE monad constructors types have changed
from

data GE a
  = FatalError [NE.NonEmpty String] (NE.NonEmpty String)
  | GenError [NE.NonEmpty String] (NE.NonEmpty String)
  | Result [NE.NonEmpty String] a

To

data GE a
  = FatalError [NE.NonEmpty String]
  | GenError [NE.NonEmpty String]
  | Result a

Checklist

• Commits in meaningful sequence and with useful messages
• Tests added or updated when needed
• CHANGELOG.md files updated for packages with externally visible changes
  
  New section is never added with the code changes. (See RELEASING.md)
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary
  
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Code formatted (use scripts/fourmolize.sh)
• Cabal files formatted (use scripts/cabal-format.sh)
• hie.yaml updated (use scripts/gen-hie.sh)
• Self-reviewed the diff