Integrate clang-tidy (take 2)

[etwoo]

Fixes #666 (follow up to premature attempt: #980)

In this PR:

1. add a default .clang-tidy file appropriate for interactive developer use
2. run clang-tidy under CI

Sidenote: I have not implemented a containerized approach because, unlike clang-format and ./tools/run-clangcldocker.sh, clang-tidy needs to know how the build is configured and what command-line arguments are passed to the compiler. My understanding is that clang-tidy typically has access to this information in one of the following ways:

• configure with -DCMAKE_CXX_CLANG_TIDY=..., and let cmake run clang-tidy for each *.cpp file, as part of the subsequent build
• configure with -DCMAKE_EXPORT_COMPILE_COMMANDS=ON to generate ./build/compile_commands.json, and then pass this information via the -p argument, like:

jq --raw-output0 .[].file ./build/compile_commands.json | xargs -0 clang-tidy -p ./build/

With either approach, containerization seems to offer little advantage, as clang-tidy is tightly coupled to the rest of the build toolchain regardless (configure-time feature detection, compiler flags, etc).

[lemire]

Running tests.