Public API for Date::added() and Date::until()

[hsivonen]

trafficstars

Temporal requires https://tc39.es/proposal-temporal/#sec-temporal-calendardateaddition and https://tc39.es/proposal-temporal/#sec-temporal-calendardatedifference , but these are unstable hidden APIs in ICU4X. For Temporal compat, we need public API surface for these.

[sffc]

Given that we currently have experimental code for this, and it ties in a lot with Temporal and the calendar work we've been doing, I think it should be a 1.x priority.

[sffc]

https://tc39.es/proposal-temporal/#sec-temporal-calendardateadd

[sffc]

Also see https://github.com/tc39/proposal-intl-era-monthcode/issues/32

[Manishearth]

We have these APIs but they are currently unstable. TO be graduated in 2.2

[Manishearth]

See also: https://github.com/unicode-org/icu4x/issues/7161