Review core/ wrappers

[pragmatrix]

To prepare for the completion of the wrappers in include/core/, I'd like to do an initial review to collect further TODOs and details about what is needed to complete them all.

Review process:

• Go through all files one by one.
  • Consider fixing TODOs.
  • Prefer impl Trait over generic parameters.
• Go over all implementation files and see if they comply to the conclusion in Try to avoid prefixes for wrapped, nested C++ types. #81.
• Use impl Trait wherever possible in all Rust files (Replace generic arguments with impl Trait wherever possible. #102).
• Implement all functions that are unimplemented! to prevent surprise bombs.
• Go through all remaining TODOs again.
• Add the get_ prefix to all functions that are not actually getters.
• Support SkFlattenable.
• Replace all enum [Name] {} static class wrappers with modules.
• Do a final review of all the changes.
• Clippy.
• Update versions (Cargo.toml & deprecated attributes) (predicted: 0.12.0)!

This PR closes #81 and closes #102.