Prior to this commit, code for scalar multiplication the module
secp256k1_main in jmbitcoin relied on direct access to the secp256k1
linked library, but the API for accessing that object has now changed in
python-bitcointx with version 1.1.5 (there is now a Secp256k1 data class
with lib and ctx entries, see the Readme of the project for details).
After this commit, we update our code for that API (but do not make
functional changes to Joinmarket itself).
Adam Gibson
Kristaps Kaupe
roshii