IDL gen improvements #1178

Job	Run time
build_and_test (rustcrypto)	5m 25s
build_and_test (rustcrypto, alloc)	5m 28s
build_and_test (rustcrypto, os)	7m 20s
build_and_test (mbedtls)	5m 43s
build_and_test (mbedtls, alloc)	5m 50s
build_and_test (mbedtls, os)	7m 29s
build_and_test (openssl)	5m 30s
build_and_test (openssl, alloc)	5m 35s
build_and_test (openssl, os)	7m 11s
	55m 31s

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