RFC: Unify pub for classes and pub for export

[nikomatsakis]

Right now classes have pub and priv sections but exported items are labeled individually. We should do the same thing, I think, for both. I'd prefer to see class items individually labeled public and private with private the default, just as with modules. Reasonable people may differ on this, however.

[marijnh]

Agreed. I also prefer the look (flat, explicit) of per-item pub/priv declarations.

[graydon]

I'm ok with this, so long as whatever chatter-reduction technique we use in modules (pub *; or whatever) also works here. Symmetry is good, verbosity not as good.

[catamorphism]

I'm in favor. Getting rid of pub/priv sections would make the parser a little simpler. (This would make #2292 obsolete if adopted.)

[nikomatsakis]

Dup of #1893