add safe API wrapper for `GetComputerNameExW` #1212

Job	Run time
rustfmt	15s
test (pinned, windows-latest, 1.72.0)	56s
test (stable, windows-latest, stable)	1m 3s
test (beta, windows-latest, beta)	51s
test (nightly, windows-latest, nightly)	43s
test (win-gnu, windows-latest, stable-x86_64-gnu)	57s
test (linux, ubuntu-latest, stable)	16s
test (macos, macos-latest, stable)	40s
	5m 41s

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