IDL gen improvements #1171

Job	Run time
build_and_test (rustcrypto)	1m 9s
build_and_test (rustcrypto, alloc)	1m 9s
build_and_test (rustcrypto, os)	1m 23s
build_and_test (mbedtls)	1m 22s
build_and_test (mbedtls, alloc)	1m 24s
build_and_test (mbedtls, os)	1m 24s
build_and_test (openssl)	1m 14s
build_and_test (openssl, alloc)	1m 25s
build_and_test (openssl, os)	1m 24s
	11m 54s

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