Code coverage (#529)

Adds `make coverage` (cargo-llvm-cov over the full workspace including
revive-llvm-builder), a new "Code coverage" book chapter that briefly
explains how to run it, and a PR workflow that runs coverage and
comments on the PR (it updates the same comment if it already exists)
when the `measure code coverage` label is applied (see e.g. [this
comment](https://github.com/paritytech/revive/pull/529#issuecomment-4966136441)).

Building instrumented LLVM and generating an LLVM C++ coverage report is
also possible, see accompanying [PR description
#575](https://github.com/paritytech/revive/pull/575) for more details.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Signed-off-by: xermicus <bigcyrill@hotmail.com>
Co-authored-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Co-authored-by: xermicus <cyrill@parity.io>
Co-authored-by: elle-j <linnea@parity.io>
Co-authored-by: LJ <81748770+elle-j@users.noreply.github.com>

.github/workflows/test-llvm-builder.yml

@@ -33,7 +33,7 @@ jobs:
       - name: Install Dependencies
         if: matrix.runner == 'parity-large'
         run: |
-          sudo apt update && sudo apt-get install -y cmake ninja-build curl git libssl-dev pkg-config clang lld musl xz-utils build-essential
+          sudo apt update && sudo apt-get install -y cmake ninja-build curl git libssl-dev pkg-config clang lld musl xz-utils build-essential zlib1g-dev
 
       - name: Install Dependencies
         if: matrix.runner == 'macos-15'