hkdf_sha256: okm is a void *.
[ccan] / ccan / crypto /
drwxr-xr-x   ..
drwxr-xr-x - hkdf_sha256
drwxr-xr-x - hmac_sha256
drwxr-xr-x - ripemd160
drwxr-xr-x - sha256
drwxr-xr-x - sha512
drwxr-xr-x - shachain
drwxr-xr-x - siphash24