Add job to competently check MSRVs

The prior workflow (now deleted) required manually specifying the packages to
check and only checked the package could compile under the stated MSRV. It
didn't verify it was actually the _minimum_ supported Rust version. The new
version finds the MSRV from scratch to check if the stated MSRV aligns.

Updates stated MSRVs accordingly.

Also removes many explicit dependencies from secq256k1 for their re-exports via
k256. Not directly relevant, just part of tidying up all the `toml`s.

.github/workflows/lint.yml

@@ -91,6 +91,99 @@ jobs:
           cargo +1.89 install cargo-machete --version =0.8.0
           cargo +1.89 machete
 
+  msrv:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac
+      - name: Verify claimed `rust-version`
+        shell: bash
+        run: |
+          cargo +1.89 install cargo-msrv --version =0.18.4
+
+          function check_msrv {
+            # We `cd` into the directory passed as the first argument, but will return to the
+            # directory called from.
+            return_to=$(pwd)
+            echo "Checking $1"
+            cd $1
+
+            # We then find the existing `rust-version` using `grep` (for the right line) and then a
+            # regex (to strip to just the major and minor version).
+            existing=$(cat ./Cargo.toml | grep "rust-version" | grep -Eo "[0-9]+\.[0-9]+")
+
+            # We then backup the `Cargo.toml`, allowing us to restore it after, saving time on future
+            # MSRV checks (as they'll benefit from immediately exiting if the queried version is less
+            # than the declared MSRV).
+            mv ./Cargo.toml ./Cargo.toml.bak
+
+            # We then use an inverted (`-v`) grep to remove the existing `rust-version` from the
+            # `Cargo.toml`, as required because else earlier versions of Rust won't even attempt to
+            # compile this crate.
+            cat ./Cargo.toml.bak | grep -v "rust-version" > Cargo.toml
+
+            # We then find the actual `rust-version` using `cargo-msrv` (again stripping to just the
+            # major and minor version).
+            actual=$(cargo msrv find --output-format minimal | grep -Eo "^[0-9]+\.[0-9]+")
+
+            # Finally, we compare the two.
+            echo "Declared rust-version: $existing"
+            echo "Actual rust-version: $actual"
+            [ $existing == $actual ]
+            result=$?
+
+            # Restore the original `Cargo.toml`.
+            rm Cargo.toml
+            mv ./Cargo.toml.bak ./Cargo.toml
+
+            # Return to the directory called from and return the result.
+            cd $return_to
+            return $result
+          }
+
+          # Check each member of the workspace
+          function check_workspace {
+            # Get the members array from the workspace's `Cargo.toml`
+            cargo_toml_lines=$(cat ./Cargo.toml | wc -l)
+            members=$(cat Cargo.toml | grep "members\ \=\ \[" -m1 -A$cargo_toml_lines | grep "]" -m1 -B$cargo_toml_lines)
+            # Parse out any comments, including comments post-fixed on the same line as an entry
+            members=$(echo "$members" | grep -Ev "^[[:space:]]+#" | grep -Ev "^[[:space:]]?$" | awk -F',' '{print $1","}')
+            # Prune `members = [` to `[` by replacing the first line with just `[`
+            members=$(echo "$members" | sed "1s/.*/\[/")
+            # Remove the trailing comma by replacing the last line's "," with ""
+            members=$(echo "$members" | sed "$(($(echo "$members" | wc -l) - 1))s/\,//")
+            # Correct the last line, which was malleated to "]," when pruning comments
+            members=$(echo "$members" | sed "$(echo "$members" | wc -l)s/\]\,/\]/")
+
+            # Don't check the patches
+            members=$(echo "$members" | grep -v "patches")
+            # Don't check the following
+            # Most of these are binaries, with the exception of the Substrate runtime which has a
+            # bespoke build pipeline
+            members=$(echo "$members" | grep -v "networks/ethereum/relayer\"")
+            members=$(echo "$members" | grep -v "message-queue\"")
+            members=$(echo "$members" | grep -v "processor/bin\"")
+            members=$(echo "$members" | grep -v "processor/bitcoin\"")
+            members=$(echo "$members" | grep -v "processor/ethereum\"")
+            members=$(echo "$members" | grep -v "processor/monero\"")
+            members=$(echo "$members" | grep -v "coordinator\"")
+            members=$(echo "$members" | grep -v "substrate/runtime\"")
+            members=$(echo "$members" | grep -v "substrate/node\"")
+            members=$(echo "$members" | grep -v "orchestration\"")
+
+            # Don't check the tests
+            members=$(echo "$members" | grep -v "mini\"")
+            members=$(echo "$members" | grep -v "tests/")
+
+            echo $members | jq -r ".[]" | while read -r member; do
+              check_msrv $member
+              correct=$?
+              if [ $correct -ne 0 ]; then
+                return $correct
+              fi
+            done
+          }
+          check_workspace
+
   slither:
     runs-on: ubuntu-latest
     steps: