Prepare for a breaking change in the Rust compiler. #110

Job	Run time
build_and_test (rustcrypto, stable)	1m 19s
build_and_test (rustcrypto, nightly)	1m 23s
build_and_test (rustcrypto, alloc, stable)	1m 57s
build_and_test (rustcrypto, alloc, nightly)	1m 44s
build_and_test (rustcrypto, os, stable)	3m 4s
build_and_test (rustcrypto, os, nightly)	3m 12s
build_and_test (mbedtls, stable)	1m 44s
build_and_test (mbedtls, nightly)	1m 39s
build_and_test (mbedtls, alloc, stable)	1m 39s
build_and_test (mbedtls, alloc, nightly)	1m 40s
build_and_test (mbedtls, os, stable)	2m 55s
build_and_test (mbedtls, os, nightly)	3m 48s
build_and_test (openssl, stable)	1m 37s
build_and_test (openssl, nightly)	1m 25s
build_and_test (openssl, alloc, stable)	1m 49s
build_and_test (openssl, alloc, nightly)	1m 25s
build_and_test (openssl, os, stable)	2m 24s
build_and_test (openssl, os, nightly)	2m 44s
	37m 28s

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