IDL gen improvements #1185

Job	Run time
build_and_test (rustcrypto)	1m 14s
build_and_test (rustcrypto, alloc)	1m 14s
build_and_test (rustcrypto, os)	1m 17s
build_and_test (mbedtls)	1m 27s
build_and_test (mbedtls, alloc)	1m 23s
build_and_test (mbedtls, os)	1m 25s
build_and_test (openssl)	1m 11s
build_and_test (openssl, alloc)	1m 11s
build_and_test (openssl, os)	1m 19s
	11m 41s

Provide feedback