Implement `Default` for sys structs

[Supreeeme]

It would be nice, if similar to ash, openxr-sys had default implementations for all of the structs, so I can not worry about the structure type and next pointer and whatnot. This would be primarily useful when writing layers. I imagine this being rather tedious to implement though. I'm considering doing it myself, but I thought I'd gather opinions first.

[Ralith]

Might not be too bad, since it should be auto-generated. If it's only useful for layers we should feature-gate it, though.

[zmerp]

It's definitely useful in case we want to write our own wrapper around openxr-sys, or use openxr-sys directly.

[Ralith]

No, that use case is handled by the existing helpers.