Superclass wrapper structs

[HertzDevil]

Resolves #68. Work to do:

• [x] Generate nested Superclass wrappers
• [x] Re-wrap abstract methods in the Superclass wrappers
• [x] Generate C++ abstract method wrappers that ignore the BgJumptables
• [x] Make superclass wrappers opt-in / opt-out via YAML config
• [x] User documentation

Some notes:

• The #superclass methods are private, however Crystal's private is closer to protected in Java so usage is the same.
• Every class that goes through Bindgen::Generator::Crystal must be a Crystal class, so the Superclass wrappers aren't structs right now. (I tried to do something like klass.origin.class? ? "class" : "struct" but it seemed to turn every Crystal wrapper class into a struct; this needs further investigation.)
• The def name : UInt8* edits merely resolve a compiler warning that will become an error in Crystal 1.0.

[Papierkorb]

So, I rebased the two PRs locally. The specs are all-green and I even got qt5.cr to run and display something in a WSL2 environment. Really cool features you added to Bindgen, Thank You! 😃

Will close this PR and then push the changes to master.

Cheers!

[docelic]

Amazing!