Implement Default for &Option

[camsteffen]

Pretty straightforward, but here is some justification:

I wasn't able to derive Default for my struct containing a &Option<MyType> because I can't implement Default for &Option<MyType> because of the orphan rule. I suppose this is an unusual case because Option<&T> is generally preferred over &Option<T>. But I think adding this is justified for a couple reasons. 1) It removes an unnecessary speed bump. 2) I think there are times when &Option<T> is preferable. In my project I am creating and passing around &Option<MyType> in a lot of places, but only actually using MyType in one place. So it's not worth the ergonomic cost of having to write build_mytype(...).as_ref() instead of &build_mytype(...) in many places.

r? libs

[jieyouxu]

r? libs-api

[camsteffen]

Looking at this more closely, this is not as obvious as I thought. There doesn't seem to be any precedent in std for implementing Default for any reference types excluding unsized types like &str. Is this a slippery slope of adding impl Default to references to all std types? Or is there an obvious solution I'm missing? Also I realized that feature(default_field_values) would be good enough for the use case I encountered. So I'm unsure, but still wonder if adding this is worth it because of the ubiquity of Option and the orphan rule. (I probably should have started a discussion instead of a PR, apologies)