Fixes building on Windows

[NicolasFlamel1]

os.rename fails on Windows if the destination already exists.

[NicolasFlamel1]

So this fixes building on Windows with the following commands since cloning the repo includes an empty secp256k1-zkp folder.

git clone https://github.com/grinventions/secp256k1-zkp-mw.git
cd secp256k1-zkp-mw
pip install .

However I just looked at the contents of the Python package with pip download secp256k1-zkp-mw and it doesn't include an empty secp256k1-zkp folder, so this might not fix installing on Windows with pip install secp256k1-zkp-mw.

Don't merge this yet. I'll keep looking into it.

[NicolasFlamel1]

I was able to get this all figured out. The Visual Studio C/C++ compiler doesn't work when files with long path names are used, and pip uses %TEMP%\pip-install\... as its default temp location when installing modules which can easily exceeds the compilers path length limitation. So to install this module on Windows you'll need to change the TEMP environmental variable to be something with a short path, like C:\temp, and also create that temp folder. This PR adds that information to the readme.

That's not all. The module also fails to install on Windows because of the absolute paths in the module's secp256k1-zkp-mw-0.1.0/secp256k1_zkp_mw.egg-info/SOURCES.txt file. So you'll need to change them from this:

/Users/marek/Development/open-source/grinventions/tmp3/secp256k1-zkp-mw/secp256k1-zkp/contrib/lax_der_parsing.c
/Users/marek/Development/open-source/grinventions/tmp3/secp256k1-zkp-mw/secp256k1-zkp/src/secp256k1.c

to this and republish the module with that change.

secp256k1-zkp/contrib/lax_der_parsing.c
secp256k1-zkp/src/secp256k1.c

This has been a great reminder of why I don't use Windows lol.

[marekyggdrasil]

Thank you @NicolasFlamel1 for investigating this. I am about to merge your changes, bump up the version and re-publish.