IDL gen improvements #1162

Job	Run time
build_and_test (rustcrypto)	1m 6s
build_and_test (rustcrypto, alloc)	1m 19s
build_and_test (rustcrypto, os)	1m 10s
build_and_test (mbedtls)	1m 20s
build_and_test (mbedtls, alloc)	1m 21s
build_and_test (mbedtls, os)	1m 21s
build_and_test (openssl)	1m 10s
build_and_test (openssl, alloc)	1m 5s
build_and_test (openssl, os)	1m 8s
	11m 0s

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