chore!: Bump to hugr-0.20 (#862)

BREAKING CHANGE: bump to hugr 0.20

---------

Co-authored-by: Mark Koch <mark.koch@quantinuum.com>
Co-authored-by: Seyon Sivarajah <seyon.sivarajah@quantinuum.com>
Co-authored-by: Agustín Borgna <agustin.borgna@quantinuum.com>
Co-authored-by: Luca Mondada <luca@mondada.net>

Author: 5 people

.github/workflows/ci.yml

@@ -226,7 +226,8 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           # Nightly is required to count doctests coverage
-          toolchain: 'nightly'
+          # TODO: Pinned due to <https://github.com/CQCL/hugr/issues/2227
+          toolchain: "nightly-2025-05-14"
           components: llvm-tools-preview
       - name: Install LLVM and Clang
         uses: KyleMayes/install-llvm-action@v2