Prefix all puri symbols.

Hi Hans,

some of the symbols from puri are prefixed.

I think this is a good idea - I made a patch (semi-automatically)
to prefix all of them.

Also the symbol uri comes from puri, I left that one alone, because
it is mostly used as a variable name.

Except in one place, which has been changed to `make-instance 'puri:uri`.

I hope you like it and will pull it upstream :)

Best
Kilian