Stable rust compatibility

[dhardy]

It turns out compatibility with stable Rust is significantly easier than last time I evaluated this.

The make_widget! macro remains nightly-only, but is now strictly a convenience macro, thus can be avoided. Most examples will continue to use it (since it is the intended way to write GUIs), but some examples will be kept compatible with stable Rust.

The clock, hello and mandlebrot examples are now compatible with stable Rust.

Specifically:

• proc_macro diagnostics are auto-enabled on nightly compilers
• an unsize feature flag was added to kas-theme (and forwarded in kas-wgpu)
• the Mandlebrot example was adjusted to avoid make_widget!