Add support for generating coverage

[maxammann]

This adds experimental support for generating coverage using LLVM and cargo-llvm-cov. Note that replacing cargo-llvm-cov would be simple. It might simplify stuff even as cargo-llvm-cov basically just invokes llvm-profdata and llvm-cov.

How to use

Suppose you have a setup like in the examples directory of the repository. Then:

1. Install cargo-llvm-cov: cargo install cargo-llvm-cov
2. Install LLVM tools (they might get installed by cargo-llvm-cov automatically, we need to test that): rustup toolchain install nightly --component llvm-tools-preview
3. Run cargo test
4. Optionally, fuzz the project.
5. Execute cargo-test-fuzz test-fuzz target --no-instrumentation --replay corpus --coverage

Coverage is generated as HTML in target/llvm-cov/html.

TODO:

• Should --coverage imply --no-instrumentation?
• Should --coverage be compatible with fuzzing? (probably not, even though it might just work)
• Make sure that llvm-tools-preview and cargo-llvm-cov get installed or promt for installation.
• On macOS I get the following warning during generation: warning: 4 functions have mismatched data
• During previous experiments I got such coverage data:
  
  I suspect this is due to the fact that test-fuzz injects code into the function-to-test here:

  test-fuzz/macro/src/lib.rs

  Line 727 in c247ff5

  #write_concretizations_and_args

  
  Either move that code somewhere else, or put it into a function, ad a no_coverage instruction and call that from the target function.

Details

The follwing commands are executes to generate coverage. We should validate them:

     Running `/Users/max/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin/llvm-profdata merge -sparse -f /Users/max/projects/examples/target/examples-profraw-list -o /Users/max/projects/examples/target/examples.profdata`
     Running `/Users/max/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin/llvm-cov show -format=html -instr-profile=/Users/max/projects/examples/target/examples.profdata -object target/debug/deps/alloc-34551e9cc1ff6b3f -object target/debug/deps/arc-bf8fdeaeed680770 -object target/debug/deps/assert-ba6618cd737ff666 -object target/debug/deps/associated_type-7ed59268473251eb -object target/debug/deps/auto_concretize_0-736514e466198bc3 -object target/debug/deps/auto_generate-7287f7108f5b00a9 -object target/debug/deps/conversion-c5a93588844a6594 -object target/debug/deps/debug-ff5617e8189a0951 -object target/debug/deps/default-79bdc830b898ec0f -object target/debug/deps/from-2bc06bf9dd96fea9 -object target/debug/deps/generic-23602c4ce1352038 -object target/debug/deps/hello_world-594474f770d22b32 -object target/debug/deps/hello_world-cff0c92d5749ded3 -object target/debug/deps/lifetime-f34199c00924285b -object target/debug/deps/manual_leak-7a7f8bfdea2a3f10 -object target/debug/deps/mut_ref-295c3cd2737a4d53 -object target/debug/deps/parse_duration-d93f2ce3d20bd782 -object target/debug/deps/qwerty-72231f74eb29ea0a -object target/debug/deps/rename-503984417115eb3f -object target/debug/deps/return_type-3b4f1b217dc76985 -object target/debug/deps/serde-26f50433033e3e5d -object target/debug/deps/test_fuzz_impl-2a426bdf99ecc53a -object target/debug/deps/unserde-4a2d01352b369939 -object target/debug/hello-world -ignore-filename-regex 'test-fuzz|/rustc/([0-9a-f]+|[0-9]+\.[0-9]+\.[0-9]+)/|^/Users/max/projects/examples(/.*)?/(tests|examples|benches)/|^/Users/max/projects/examples/target($|/)|^/Users/max/\.cargo/(registry|git)/|^/Users/max/\.rustup/toolchains($|/)' -show-instantiations=false -show-line-counts-or-regions -show-expansions -show-branches=count -Xdemangler=/Users/max/.cargo/bin/cargo-llvm-cov -Xdemangler=llvm-cov -Xdemangler=demangle -output-dir=/Users/max/projects/examples/target/llvm-cov/html`

fixes #293