feat(coverage): Reduce coverage false-negatives using grcov

Previously, we would report some tag style macros such as
`#[repr(packed)]` as being uncovered.

We are now also including doctests in our coverage reports.

Finally, a new script `coverage_report.sh` makes coverage checking
easier.

flake.nix

@@ -121,6 +121,7 @@
               proverif-patched
               inputs.fenix.packages.${system}.complete.toolchain
               pkgs.cargo-llvm-cov
+              pkgs.grcov
             ];
           };
           devShells.coverage = pkgs.mkShell {
@@ -128,6 +129,7 @@
             nativeBuildInputs = [
               inputs.fenix.packages.${system}.complete.toolchain
               pkgs.cargo-llvm-cov
+              pkgs.grcov
             ];
           };