We can have more than one; eg we might offer both bech32 and a p2sh
address, and in future we might offer v1 segwit, etc.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
I didn't convert all tests: they can still use a standalone context.
It's just marginally more efficient to share the libwally one for all
our daemons which link against it anyway.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>