CWEB examples

Run './downloadable-programs.sh --get' to curl tons of CWEB example
programmes. For the 'sat' programmes, there's also an archive
'http://www-cs-faculty.stanford.edu/~uno/programs/SATexamples.tgz', but
that's simply _huge_ (around 60 MiB), so the script does _not_ pull this
every time (nor do I push this on Github). The same applies to 24 MiB
'http://www-cs-faculty.stanford.edu/~uno/programs/boolchains.tgz'.