Add proper CI testing coverage and optimize it for speed.

[xStrom]

I spent some time really reading through the cargo docs and inspecting our workspace structure. This PR is the result of that. It covers more code and is faster.

• Made sure everything is tested on all target combinations. There were some holes with examples, including book examples. Also nightly and wasm testing were limited.
• Removed redundant cargo rustc steps as cargo clippy already compiles the code and fails on the same warnings. I verified this and it's stated in the clippy readme:

Note that adding -D warnings will cause your build to fail if any warnings are found in your code. That includes warnings found by rustc (e.g. dead_code, etc.).

• Brought X11 testing into the main job as an end phase on Ubuntu, because it shares a lot of dependencies that are already built.
• Added various comments to the CI file for future editors.

[cmyr]

Wonderful, the problem with this stuff is definitely maintenance; it seems to consistently degrade over any serious set of changes. Hopefully we don't have too many additional targets coming in the near future, though? 😬

[cmyr]

Looks good on a skim. Thanks for taking this on!