Implement secp256k1_ecdsa_signature_serialize and int secp256k1_ecdsa_signature_parse

[rustyrussell]

Non-DER versions, which just fill in 64 bits in some canonical order.

Alternately: make the opaque structure a canonical order.

[sipa]

See my laxder branch, it also adds those. I'll PR when I either decide to split it off from the DER rework, or tests for that are finished.

[rustyrussell]

Yep, looks good! Please document the serialize format (at a glance, it's 32 byte unsigned BE r value followed by 32 byte unsigned BE s value).

I'd still like a "is_normalized" or "is_canonical" for the s-value test BTW, but I can implement my own with these.

[sipa]

Working on those!

[sipa]

Fixed by #334.