Tracking issue for RFC #2145: Type privacy and private-in-public lints

[aturon]

This is a tracking issue for the RFC "Type privacy and private-in-public lints " (rust-lang/rfcs#2145).

Steps:

• Implement the RFC (cc @petrochenkov )
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Unresolved questions:

• It's not fully clear if the restriction for associated type definitions required for
  type privacy soundness, or it's just a workaround for a technical difficulty.

• Interactions between macros 2.0 and the notions of reachability / effective
  visibility used for the lints are unclear.