Run external tool for calculating wt-144

[dagurval]

This change exports current state, then executes an external program to calculate wt-144 target. I use this for testing the C++ implementation.

I use this together with this bitcoin-abc patch: https://gist.github.com/dagurval/d93f0d14d0a20440ef32ef593dd87a42

To run:

• Apply patch to bitcoin-abc
• Compile by running scons
• Symlink cashwork executable to path with the python program.

[dgenr8]

@dagurval Can you post a brief summary of the results of the comparison? Does the c++ implementation match the python results?

[dagurval]

@dgenr8 I've been running this continuously with random seeds for a day. No asserts have triggered so far, meaning that the implementations match.