Add new_buffer_private for metal #806

Job	Run time
Test Suite (windows-latest, stable)	35s
Test Suite (macOS-latest, stable)	35s
Check (ubuntu-latest, stable)	1m 4s
Test Suite (ubuntu-latest, stable)	39s
Rustfmt	7s
Check (windows-latest, stable)	1m 0s
Check (macOS-latest, stable)	1m 0s
Clippy	34s
	5m 34s

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