Removes pin to nightly and removes the old -Zprofile runs and grcov based coverage generation in favor of -C source-instrumentation which is in stable.
This updates the CI to install llvm-tools-preview from rustup and install a nice wrapper for running source instrumentation called cargo-llvm-cov. We use this wrapper to generate codecov JSON to get region coverage since LCOV style output from source coverage in Rust/LLVM doesn't output branch coverage.
Cf. https://github.com/amazon-ion/ion-rust/pull/538 :