Setup cross toolchain for miri #180

Job	Run time
fmt	17s
Clippy	22s
feature checks	27s
docs	22s
Fuzz	31s
Miri (x86_64-unknown-linux-gnu, --no-default-features)	59s
Miri (x86_64-unknown-linux-gnu)	1m 13s
Miri (x86_64-unknown-linux-gnu, --all-features)	1m 8s
Miri (aarch64-unknown-linux-gnu, --no-default-features)	2m 6s
Miri (aarch64-unknown-linux-gnu)	1m 11s
Miri (aarch64-unknown-linux-gnu, --all-features)	1m 27s
Outdated	9s
Test (x86_64-unknown-linux-gnu, nightly)	22s
Test (x86_64-unknown-linux-gnu, beta)	23s
Test (x86_64-unknown-linux-gnu, stable)	27s
Test (x86_64-unknown-linux-gnu, 1.64)	35s
Test (aarch64-unknown-linux-gnu, nightly)	49s
Test (aarch64-unknown-linux-gnu, beta)	1m 2s
Test (aarch64-unknown-linux-gnu, stable)	1m 1s
Test (aarch64-unknown-linux-gnu, 1.64)	1m 4s
	15m 55s

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