Hacks for usage with stable Rust

[dhardy]

This puts proc-macro diagnostics behind a feature gate, and uses proc-macro-hack to avoid a requirement on proc_macro_hygiene.

Fixes #25.

I am unsure whether to merge this. As nice as it is to be able to use stable Rust, it hides macro diagnostics by default, uglifies the code-base (especially with make_widget having to be exported from the root of kas), and examples need high macro recursion limits:

// calculator.rs
#![recursion_limit = "1024"]

[dhardy]

This is now completely obsolete (thanks Rust 1.45 🎉).