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.

coverage_report.sh

@@ -0,0 +1,44 @@
+#! /usr/bin/env bash
+
+set -e -o pipefail
+
+OUTPUT_DIR="target/grcov"
+
+log() {
+  echo >&2 "$@"
+}
+
+exc() {
+  echo '$' "$@"
+  "$@"
+}
+
+main() {
+  exc cd "$(dirname "$0")"
+
+  local open="0"
+  if [[ "$1" == "--open" ]]; then
+    open="1"
+  fi
+
+  exc cargo llvm-cov --all-features --workspace --doctests
+
+  exc rm -rf "${OUTPUT_DIR}"
+  exc mkdir -p "${OUTPUT_DIR}"
+  exc grcov target/llvm-cov-target/ --llvm  -s . --branch \
+    --binary-path ./target/llvm-cov-target/debug/deps \
+    --ignore-not-existing --ignore '../*' --ignore "/*" \
+    --excl-line '^\s*#\[(derive|repr)\(' \
+    -t lcov,html,markdown -o "${OUTPUT_DIR}"
+
+  if (( "${open}" == 1 )); then
+    xdg-open "${PWD}/${OUTPUT_DIR}/html/index.html"
+  fi
+
+  log ""
+  log "Generated reports in \"${PWD}/${OUTPUT_DIR}\"."
+  log "Open \"${PWD}/${OUTPUT_DIR}/html/index.html\" to view HTML report."
+  log ""
+}
+
+main "$@"