Function secp256k1_ecdh
pub unsafe extern "C" fn secp256k1_ecdh(
cx: *const Context,
output: *mut u8,
pubkey: *const PublicKey,
seckey: *const u8,
hashfp: Option<unsafe extern "C" fn(_: *mut u8, _: *const u8, _: *const u8, _: *mut c_void) -> i32>,
data: *mut c_void,
) -> i32