Support `-Zbuild-std` #688

Job	Run time
check clippy	22s
build-std (ubuntu-latest, x86_64-unknown-linux-gnu)	1m 39s
check rustfmt	10s
tests (ubuntu-latest, x86_64-unknown-linux-gnu)	3m 47s
build-std (macos-latest, x86_64-apple-darwin)	1m 41s
tests (macos-latest, x86_64-apple-darwin)	3m 48s
build-std (windows-latest, i686-pc-windows-msvc)	3m 30s
tests (windows-latest, i686-pc-windows-msvc)	7m 27s
	22m 24s

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