Cleanup `Now` API in favor of system defined implementations #589

Job	Run time
Lint	29s
Test FFI (stable, ubuntu-latest)	1m 38s
Documentation	1m 24s
Test (stable, ubuntu-latest)	31s
MSRV	1m 9s
Test (stable, macos-latest)	1m 43s
Test (stable, windows-latest)	1m 57s
	8m 51s

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