build: Add script for reporting coverage methods

It's named `coverage.sh` and uses `cargo-llvm-cov`[0].

[0] https://github.com/taiki-e/cargo-llvm-cov

scripts/coverage.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env sh
+
+. "./scripts/devenv.sh" || { echo >&2 "Failed to source devenv.sh. Aborting."; exit 1; }
+
+set -eux
+
+export RUST_MIN_STACK=8388608
+export RUSTFLAGS="-D warnings"
+
+ROOT_DIR=$(git rev-parse --show-toplevel)
+
+cargo llvm-cov \
+  --all-targets --workspace \
+  --exclude light-concurrent-merkle-tree \
+  --exclude photon-api \
+  --exclude forester \
+  --html
+cargo llvm-cov \
+  --all-targets \
+  --package light-concurrent-merkle-tree \
+  --html \
+  --output-dir "${ROOT_DIR}/target/llvm-cov-mt"