Implement chrdev

[alex]

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[alex]

@geofft Ok. So here are the open questions:

• FileOperations probably needs an fn open(<something) -> Self right? Should our function pointer then Box<Self> and stick it in file->private? Is file->private ever anything besides a pointer? Do users ever need more control than that?
• Every method on FileOperations can be called concurrently with one another, right? We probably need to make all the methods take &self, requires FileOperations to be Sync, and tell people to put locks around their shit if they want to do stuff, right? We shouldn't provide locking for them, I don't think?
• Do you have any objections to the API I've sketched out in the PR? Just making the vtable explicit seemed like the easiest thing.

[alex]

Oh... also the lifetimes around DeviceNumberRegion and DeviceRegistration are impossible.

[alex]

This is now rebased on top of #107, that should be reviewed/merged first.