A tiny first step: I added sha256 chechsums for all downloads created by the Pharo6 build process

This step:

now creates .sha256.txt files, e.g for the mac:

This allows to check that downloads from the file server are indeed the same files that the build server created.

As I said, just a very first step.

TODO:

- pgp signatures 

- insert into website

- SSL for files.pharo.org

- do it Pharo7

- ���.

So: more to come!

Marcus