Remove botan dependency + static builds

Remove dependency on botan, use randombytes from libsodium instead. Add some cmake code to allow static builds to easily be toggled on/off and work in both cases.
This time without any extra commits or other random crapcode I was still doing testing on, nice and clean :)

**Big fat warning**: Currently, the examples don't work for me, some of the streams hang each time, as I don't have botan around at all, I can't determine if this is due to :windows: or due to me removing botan from the mix and using libsodium's randombytes (which I doubt).

Could you check and see if the examples still run on your end ?
Please use two spaces to indent, not tabs (also elsewhere).