[WIP] widget split + 2018 edition

[Abendstolz]

This Pull request is WIP!

I've:

1. Split out most non primitive widgets from core to it's own crate, named conrod_widgets
2. Migrated conrod_derive, conrod_core and (new) conrod_widgets to rust 2018 edition

Tested with all_winit_vulkano in backends/vulkan

Todo:

• Talk with @mitchmindtree to clarify whether rust 2018 edition is even okay for conrod
• Test and fix other backends
• Think about splitting out remaining non-primitive widgets from conrod_core (bordered_rectangle, button, rounded_rectangle)

[jkelleyrtp]

Is there still a desire to separate the widgets from conrod core? I'd love to see this implemented to get better templates on how to create custom widgets.

[mitchmindtree]

Yes I'd still like to see this happen, but won't get the chance to do this myself anytime soon. This PR looks like it got fairly close though!

Re 2018 - maybe it's worth opening a new issue about this for visibility to other users. I personally don't mind too much, and think it would be nice to update to 2018, but it would be nice to see that happen in a separate PR either before or after this one.

Also another thing that crosses my mind, it might be nice to use conrod_widgets as widgets; in the examples?

[Abendstolz]

As I opened this PR half a year ago let me say that I am still willing to put work into it, but only if I am sure this gets enough attention and merged (after incorporating any feedback of course) in time. Maybe conrod needs more maintainers? (*)

I agree that a split of the work regarding 2018 edition and splitting the widget out makes sense, I did that while I was diving through the codebase and it probably resulted in too many changes for one PR.

(*) Note that this is not meant to be offensive in any way whatsoever