autoimpl; revise widget macros

[dhardy]

This PR revisits how widgets are implemented, also adding the autoimpl attribute-macro. WIP.

[dhardy]

There's still quite a bit to do for this PR:

• make_widget should not invoke widget
• eliminate core_data methods?
• use quote_spanned when generating impls etc
• implement Default?

Scratchboard for syntax ideas: https://gist.github.com/dhardy/97efb394311b8b9687c0d817e95654b9

[dhardy]

This is part of a series of changes aimed at improving the way widgets are constructed. It is not finished, but does achieve:

• replace derive(Widget) with widget! { ... } macro
• autodetection of whether widget traits like Layout are implemented directly
• macro sugar: impl Trait for Self syntax — use of Self to imply struct generics and bounds, with optional extras (this may be brought to a stand-alone crate later)
• new macro: autoimpl — more capable variant of derive (also may be brought to a stand-alone crate later)
• fix required by recent nightly compilers