feat: mman `NonNull` #28

Job	Run time
tier3 (x86_64-unknown-dragonfly)	58s
tier3 (x86_64-unknown-openbsd)	30s
macos	34s
minver	12s
tier3 (armv7-unknown-linux-uclibceabihf)	57s
linux_native_builds (x86_64-unknown-linux-gnu)	1m 36s
linux_native_builds (x86_64-unknown-linux-musl)	1m 28s
tier3 (x86_64-unknown-haiku)	46s
rust_stable	1m 20s
rustfmt	6s
redox	0s
cross	0s
cross_compiling	0s
	8m 27s

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