IDL gen improvements #1155

Job	Run time
build_and_test (rustcrypto)	4m 57s
build_and_test (rustcrypto, alloc)	4m 49s
build_and_test (rustcrypto, os)	6m 40s
build_and_test (mbedtls)	5m 18s
build_and_test (mbedtls, alloc)	5m 6s
build_and_test (mbedtls, os)	6m 48s
build_and_test (openssl)	4m 42s
build_and_test (openssl, alloc)	4m 41s
build_and_test (openssl, os)	6m 31s
	49m 32s

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