Stability story of compiler flags

[huonw]

There's a pile of weird and wonderful flags for rustc and we should have some conscious story about them and their stability, even if it's just "they're all unstable".

[pnkfelix]

P-backcompat-libs, 1.0.

[aturon]

Nominating: we need to consider whether this is essential for beta.

[pnkfelix]

Assigning 1.0-beta milestone.