auto merge of #15014 : brson/rust/all-crates-experimental, r=cmr

This creates a stability baseline for all crates that we distribute that are not `std`. In general, all library code must start as experimental and progress in stages to become stable.

diff --cc src/libgreen/lib.rs
Simple merge

diff --cc src/libnative/lib.rs
Simple merge

diff --cc src/libtest/lib.rs
Simple merge

diff --cc src/libuuid/lib.rs
Simple merge