Move surfman out of webxr-api

[jdm]

Having surfman in the public api of webxr-api means that we rebuild some of the largest parts of Servo when changing surfman. These changes use generics to remove that dependency. It's not a perfect end result; it would be conceptually cleaner to push the Surface/SwapChains/SwapChain implementations into Servo itself, but that quickly runs into more complicated type shenanigans without actually yielding a significantly better compilation path.

[jdm]

You can see the Servo changes in servo/servo@master...jdm:less-rebuild.

[asajeffrey]

My attempt at doing this, which is very similar but avoids some issues with the orphan rule by putting the API in surfman-chains-api... https://github.com/asajeffrey/webxr/tree/split-surfman-chains-api-and-impl and https://github.com/asajeffrey/surfman-chains/tree/split-api-and-impl

[asajeffrey]

I've not done the matching changes to servo yet, but they should be pretty straightforward.

[asajeffrey]

The matching servo changes which are a lot like yours! https://github.com/asajeffrey/servo/tree/split-surfman-chains-api-and-impl

[Manishearth]

I'm okay with both ways of doing it.

Note that if we take Alan's approach, you will have to only patch on surfman-chains and not surfman-chains-api when using patch, and edit surfman-chains/Cargo.toml to not have a path dependency, otherwise it will end up clobbering a lot of crates.

[asajeffrey]

Submitted #91.

[bors-servo]

☔ The latest upstream changes (presumably #91) made this pull request unmergeable. Please resolve the merge conflicts.

[asajeffrey]

Closing in favour of #91. Me me me it's my code by me!