This fixes block parsing on testnet; specifically, non-standard tx versions.
We hit a type bug in libwally (wallt_get_secp_context()) which I had to
work around for the moment, and the updated libsecp adds an optional hash
function arg to the ECDH function.
Fixes: #2563
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Rusty Russell