Roald/implement layers

[RmStorm]

Hello hello, I'm back with a huge PR this time. I started using this crate to prototype some shader stuff and I got completely carried away honestly.. So uhm.. I know it's poor form to launch giant PR's like this one at an open source project :sweat_smile: very sorry about that!

I'm putting this PR in draft mode because I think we need to discuss a bunch of things if you are to consider merging it. I hope you can find some time to engage! ~~I cleaned up the work here just a little bit and divided it into 3 commits.~~

~~- One that is pure admin crap to get examples running (deleting and changing Cargo.toml's and Trunk.toml files).~~ ~~- Than a commit with a complete refactoring of the layer's and a bunch of other changes sprinkled in, if we actually get to the point where we think the work is worth merging I will obviously clean this up much more.~~ ~~- And the final commit contains example updates.~~ This PR is now gated behind: https://github.com/yukinarit/mapbox-gl-rs/pull/65 that basically gets all the examples running again some of which were broken. It also fixes other minor issues I ran into along the way.

The main attraction is basically support for custom webgl layers and strongly typed layout and paint objects. Take a quick look at this screen cap for the coolness of the strong typing! autocomplete!