Return the extension properties for constructing Duration from numbers,
but this time place them in Duration.Companion rather than on top-level.
- provide the new set of construction extension properties
in companion of Duration
- leave top-level extension properties experimental,
increase their deprecation level to ERROR,
propose the extension properties in companion instead.
- leave Duration companion static factory functions experimental,
deprecate them and propose the extension properties in companion instead.
Introduce Duration companion functions to convert numbers to Duration.
Deprecate number extension properties and propose to use these
new functions instead.
Ilya Gorbunov