Update wgpu to 0.6

[yutannihilation]

I tried to create a pull request for this, but it seems too difficult to me... So, let me just file an issue.(My failed attempt is here: https://github.com/nannou-org/nannou/compare/master...yutannihilation:poc/wgpu-0.6.0_tmp?expand=1)

Some notes:

• conrod needs to be updated. I created a pull request for this: Update wgpu to 0.6 PistonDevelopers/conrod#1385 (review)
• Since wgpu implemented new map_async() in Use the new map-async gfx-rs/wgpu-rs#344 and New map_async logic gfx-rs/wgpu#675, there's no BufferReadMapping anymore. Maybe image.rs needs some refactoring?
• array_layer is moved to other place in Implement write_texture gfx-rs/wgpu#666. I couldn't figure out how this is passed around in nannou, but I hope this shouldn't be very hard to fix.
• I needed to add tons of explicit lifetimes as more structs requires it. Not sure if this is just my skill about lifetime is very poor...

[mitchmindtree]

Thanks for diving in @yutannihilation and for sharing what you learned!

I needed to add tons of explicit lifetimes as more structs requires it.

It looks like you're not the only one to have run into this - I added a comment about addressing this on the active PR here in case you're interested.

[yutannihilation]

Thanks! Yeah, your comment on #655 is really helpful :)