Function indy_sys::crypto::indy_crypto_sign

#[no_mangle]
pub unsafe extern "C" fn indy_crypto_sign(
    command_handle: Handle, 
    wallet_handle: Handle, 
    signer_vk: CString, 
    message_raw: BString, 
    message_len: u32, 
    cb: Option<ResponseSliceCB>
) -> Error