Small bugfix

A small bug fixed on this release. Now tainted-ness infects from self to newly-created string with undump.

s = 'foo'.taint # s is tainted
t = s.undump # t is also tainted for now!
puts t.tainted? #=> true

String#undump completelly rewritten in C

This is the first release of String#undump as a C extension library.
(Formally called as String#undump_roughly)

Now 9x or more faster than eval(dumped_string) 🎉

Enjoy!