example/fannkuch.pn: accept cmdline arg, default 11

to compare timings to perl5,perl6 and python.
see http://rosettacode.org/wiki/Topswops#.7B.7Bheader.7CPotion.7D.7D
example/almost_prime.pn added

see http://rosettacode.org/wiki/Almost_prime
converted from the C version
example/palindrome.pn added

See http://rosettacode.org/wiki/Palindrome_detection#Potion
we rather use: s ord(index) !=
than s(index) cmp ...) != 0