More efficient conversion to affine when trivial.

I.e. when Z = 1, convert back to affine directly without bothering to
calculate  Z² and Z³. Point multiplication down to 1.8 seconds.
Lookup file for doubling base point in secp256k1

Unfortunately loading and parsing the plist file takes forever. The
test takes 8.45 seconds. I skip the multiplication loop it takes 9.146
seconds (which indeed is longer…).