Reconsider default-public struct fields

[brson]

Not sure if there's an issue for this, but I continue to see pull requests to hide incorrectly-public struct fields. I realize this has been beaten to death, but I still feel this is the wrong default that will continue to bite us. Nominating.

[brson]

When we close this we should add an explanation to the design faq (#4047) to cement it.

[sfackler]

There's some discussion of this in #8122.

[pnkfelix]

I'm still in favor of the pub: solution I proposed in a comment from #8122.

(I'm only talking about implementing it with respect to structs; further comments from others on #8122 discussed generalizing it to handle enum and struct tuples as well, but its not obvious to be whether such generalization is a good idea.)

[pnkfelix]

Accepted for 1.0, P-backcompat-lang.