Consider moving Instant and Clock to the standard library

[dkhalanskyjb]

Obtaining the current instant is often needed, as is doing some Duration-based operations on it. Instant is useful even when the rest of the library isn't: you don't need the timezone functionality to log human-readable timestamps (#121 (comment)) or represent file access and modification times (okio), or sort events, etc.

With this change, the kotlinx-datetime library would provide calendar- and timezone-aware operations, whereas Instant, Duration, Clock, and TimeSource would work with just "the flow of time" as seen by the computer.

• The parsing and formatting facilities for Instant should probably stay in the kotlinx-datetime library, as they aren't very meaningful without the notion of a calendar.
• InstantIso8601Serializer and InstantComponentSerializer would probably enter the kotlinx-serialization library in some form.