Tag public std interfaces with stability index

[brson]

I intend to start tracking the 'stability index' of public interfaces in standard. Since most is still 'unstable' this won't require much annotation at first.

Stability index:

• Deprecated,
• Experimental,
• Unstable,
• Stable,
• Frozen,
• Locked,

[brson]

Nominating for backward compat or well covered.

[brson]

cc #6508

[huonw]

Triage: no progress that I know of, still relevant.

[catamorphism]

Accepted for milestone 2, backwards-compatible

[huonw]

#8921 landed basic support, but there are a few issues to resolve: #8961, #8962 and #8965.