This is a bit more awkward for large structures, but avoids
indirection for the simpler ones (I copied the structures for the test
code, however). We also remove explicit padding.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changed the generation of messages so that we pass in a reference to the
secp256k1_context if it is required by the underlying primitive
function. This gets rid of the global `secp256k1_ctx` variable and
adheres closer to how we've been handing in the context so far.
