fix removed reexported imports from mtl and change stackage lts to 21.12

Hi, trying out building yi with stack from source (failed using cabal install, and stack install, from readme), i updated the stackage snapshot to lts-21.12 (ghc-9.4.7) and found some missing exports in mtl-2.2.2. This is a fix for that.

I suppose the cabal bounds should be updated, in yi/yi.cabal. Though because I don't use cabal, I don't know how to build it with it. I'd happily help to get it updated as well.

build details with cabal

``` cabal v2-build yi:yi Resolving dependencies... Error: cabal: Could not resolve dependencies: [__0] trying: yi-0.19.0 (user goal) [__1] trying: yi-rope-0.11 (dependency of yi) [__2] trying: text-2.0.2/installed-2.0.2 (dependency of yi-rope) [__3] trying: base-4.17.2.0/installed-4.17.2.0 (dependency of yi) [__4] trying: yi-frontend-pango-0.19.1 (user goal) [__5] next goal: gtk (dependency of yi-frontend-pango) [__5] rejecting: gtk-0.15.8 (conflict: pkg-config package gtk+-2.0-any, not found in the pkg-config database) [__5] rejecting: gtk-0.15.7 (conflict: text==2.0.2/installed-2.0.2, gtk => text>=0.11.0.6 && <1.3) [__5] skipping: gtk-0.15.5, gtk-0.15.4, gtk-0.15.3, gtk-0.15.2, gtk-0.15.1, gtk-0.15.0, gtk-0.14.7, gtk-0.14.6, gtk-0.14.5, gtk-0.14.4, gtk-0.14.3, gtk-0.14.2, gtk-0.13.9, gtk-0.13.8.1, gtk-0.13.8, gtk-0.13.7, gtk-0.13.6, gtk-0.13.4, gtk-0.13.3, gtk-0.13.2, gtk-0.13.1, gtk-0.13.0.3, gtk-0.13.0.2, gtk-0.13.0.1, gtk-0.13.0.0 (has the same characteristics that caused the previous version to fail: excludes 'text' version 2.0.2) [__5] rejecting: gtk-0.12.5.7 (conflict: yi-frontend-pango => gtk>=0.13 && <0.16) [__5] skipping: gtk-0.12.5.6, gtk-0.12.5.5, gtk-0.12.5.4, gtk-0.12.5.3, gtk-0.12.5.2, gtk-0.12.5.1, gtk-0.12.5.0, gtk-0.12.4, gtk-0.12.3.1, gtk-0.12.3, gtk-0.12.2, gtk-0.12.1, gtk-0.12.0, gtk-0.11.2, gtk-0.11.1, gtk-0.11.0 (has the same characteristics that caused the previous version to fail: excluded by constraint '>=0.13 && <0.16' from 'yi-frontend-pango') [__5] rejecting: gtk-0.14.10 (conflict: text==2.0.2/installed-2.0.2, gtk => text>=0.11.0.6 && <1.3) [__5] skipping: gtk-0.14.9 (has the same characteristics that caused the previous version to fail: excludes 'text' version 2.0.2) [__5] fail (backjumping, conflict set: gtk, text, yi-frontend-pango) After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: text, yi-rope, yi, gtk, base, yi-frontend-pango Try running with --minimize-conflict-set to improve the error message. ```

also, it seems some test output files were written, though i don't know what to do with them, they seem to do something weird when i try to commit them

https://github.com/yi-editor/yi/assets/79347623/b5692ac1-716e-44b8-9fa5-22e7f0e6f205

yi-editor / yi

fix removed reexported imports from mtl and change stackage lts to 21.12 #1134