Add methods for unwrapping Result<T, !> / Result<!, E>

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One of the main intended uses for the ! type is to be able construct the types Result<T, !> and Result<!, E>. Right now, if you have r: Result<T, !> there are several ways to unpack it's inner T, but none of them are ideal:

• r.unwrap(): unwrap is scary and should be avoided. This is also fragile if someone later changes the error type to something other than !.
• r.unwrap_or_else(|e| e): Kinda confusing.
• let Ok(val) = r;: Requires a separate statement. (Also doesn't currently work).
• match r { Ok(v) => v, Err(e) => e }: Both long and kinda confusing.
• match r { Ok(v) => v }: Not as nice as a method on r. (Also doesn't currently work).

The void crate adds the methods void_unwrap and void_unwrap_err to Result<T, Void> and Result<Void, T>. I think methods like these should be added to libcore as well. Perhaps we could call them always_ok() and always_err().

Start by reading the issue's examples for Result<T, !> and Result<!, E>, then compare the void crate's void_unwrap and void_unwrap_err methods. The proposed API names are still open; done would require settling the method design and adding the corresponding libcore behavior.