Fix imprecise Result path

[pierzchalski]

It's a common pattern in large crates to define a custom Error type, then define an alias type Result<T> = std::result::Result<T, Error>. Unfortunately, this proc macro referred to Result, not ::std::result::Result, so it would resolve to the custom type.

[ihrwein]

It took me a while to respond to this, but thank you.