Properly implement UserSpacePtr

[todo]

UserSpacePtr right now is just a glorified, horribly unsafe reference. We should change its interface to provide a lot more safety. It should have a get function returning a Result<T, UserspaceError> that copies the underlying type, returning an error if the underlying pointer is invalid (at least if it points to kernel memory. Maybe also if it is not mapped?).
We also have to handle unsized types. Maybe have a get_ref() which returns a &T? Maybe don't allow unsized types? I should check how Horizon/NX deals with it in sendsyncrequest (that's the only place that allows huge data afaik)

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https://github.com/roblabla42/KFS/blob/adee47274ba5c0dc0ef112ce8933becd63d3da77/kernel/src/mem.rs#L199-L209

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This issue was generated by todo based on a TODO comment in adee472 when #90 was merged. cc @roblabla.

[roblabla]

See #231