Generate mruby bindings locally and check them in to the repository instead of during build time

[lopopolo]

Check in the outputs of running bindgen on mruby and the associated C extensions required for building artichoke-backend.

bindgen is a heavy dependency with many transitives that don't already show up in Artichoke's Cargo.lock file (e.g. clang-sys). This was the motivation for replacing a Rust dev-dependency on bindgen with a requirement that the bindgen CLI be available ambiently in PATH when building Artichoke:

• Refactor artichoke-backend build to use bindgen executable instead of dev-dependency #1300

Because bindgen is available by default in GitHub Actions runners and this requirement was able to be documented in CONTRIBUTING.md, this seemed like an acceptable loss of build hermeticity. Additionally, relying on pre-built bindgen sped up clean build compilation time significantly.

The maintenance burden of this approach has been increasing recently as the bindgen CLI interface has undergone more rapid iteration:

• Fix artichoke-backend build when bindgen is not globally installed #1312
• Parallelize cc and bindgen steps in backend build script #2117
• Only install bindgen if it is absent #2197
• Parameterize and upgrade bindgen version docker-artichoke-nightly#125
• Install bindgen from bindgen-cli package #2222
• Install bindgen from bindgen-cli package docker-artichoke-nightly#127
• Add BINDGEN_PACKAGE build arg to README docker-artichoke-nightly#128
• Upgrade bindgen in artichoke-backend build.rs to 0.64.0 #2496
• Upgrade bindgen-cli to 0.64.0 docker-artichoke-nightly#147
• Upgrade artichoke-backend build script to bindgen 0.65.1 #2503
• Upgrade bindgen to 0.65.1 docker-artichoke-nightly#149

Instead, add a bindgen Rake task (invokable as bundle exec rake bindgen) which uses the bindgen CLI on a developer laptop to generate the bindings once and dumps them into the source tree to be checked in.

This allows removing a bunch of code in artichoke-backend's build.rs and makes the code authorship experience better by improving browsing of generated bindings, ensuring goto definition works, and reducing build times.

The reduction in runtime of artichoke-backend's build.rs means the long poles for Artichoke compilation are tzdb and artichoke-backend (both of these crates have a huge amount of code):

[lopopolo]

markdown link check job failure is a flake on dependabot.com

[lopopolo]

Related change to remove bindgen from Docker nightly build containers is here:

• Remove bindgen tool from build phase of Dockerfiles docker-artichoke-nightly#151

[lopopolo]

playground is busted now due to these changes:

https://github.com/artichoke/playground/actions/runs/4792255663/jobs/8523565662

--- stderr
  error: unexpected argument '--size_t-is-usize' found

    tip: a similar argument exists: '--no-size_t-is-usize'

  Usage: bindgen [FLAGS] [OPTIONS] [HEADER] -- [CLANG_ARGS]...

  For more information, try '--help'.
  thread '<unnamed>' panicked at 'bindgen failed', /home/runner/.cargo/git/checkouts/artichoke-c12ffc01676ba375/fbbe649/artichoke-backend/build.rs:315:9
  stack backtrace:
     0: rust_begin_unwind
               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:584:5
     1: core::panicking::panic_fmt
               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/panicking.rs:142:14
     2: build_script_build::libs::bindgen
               at ./build.rs:315:9
     3: build_script_build::libs::build::{{closure}}::{{closure}}
               at ./build.rs:328:17
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
  thread 'main' panicked at 'a scoped thread panicked', /home/runner/.cargo/git/checkouts/artichoke-c12ffc01676ba375/fbbe649/artichoke-backend/build.rs:319:9
  stack backtrace:
     0: rust_begin_unwind
               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:584:5
     1: core::panicking::panic_fmt
               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/panicking.rs:142:14
     2: std::thread::scoped::scope
               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/thread/scoped.rs:157:13
     3: build_script_build::libs::build
               at ./build.rs:319:9
     4: build_script_build::main
               at ./build.rs:361:5
     5: core::ops::function::FnOnce::call_once
               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/ops/function.rs:248:5
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[lopopolo]

this busted artichoke builds on 32 bit architectures, in particular wasm32-unknown-emscripten.