IDL gen improvements #1176

Job	Run time
build_and_test (rustcrypto)	1m 40s
build_and_test (rustcrypto, alloc)	1m 26s
build_and_test (rustcrypto, os)	1m 35s
build_and_test (mbedtls)	1m 40s
build_and_test (mbedtls, alloc)	1m 41s
build_and_test (mbedtls, os)	1m 40s
build_and_test (openssl)	1m 39s
build_and_test (openssl, alloc)	1m 26s
build_and_test (openssl, os)	1m 38s
	14m 25s

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