delete the guts crate for now

I haven't developed this experimental code any further since I pushed
it, and it's not good for unused code to sit around. If I ever get back
to this project I might revert this deletion.

.github/workflows/ci.yml

@@ -131,17 +131,6 @@ jobs:
       run: cargo test
       working-directory: ./reference_impl
 
-    # the new guts crate
-    - name: guts test
-      run: cargo test --all-features
-      working-directory: ./rust/guts
-    - name: guts no_std build
-      run: cargo build --no-default-features
-      working-directory: ./rust/guts
-    - name: guts no_std test  # note that rust/guts/src/test.rs still uses libstd
-      run: cargo test --no-default-features
-      working-directory: ./rust/guts
-
   b3sum_tests:
     name: b3sum ${{ matrix.target.name }} ${{ matrix.channel }}
     runs-on: ${{ matrix.target.os }}