Change to object-oriented tests and documentation.
Rm old "DESTROY". Rm "use_ok". Move documentation to separate file.

I have removed "DESTROY" from PNG.pm.tmpl (did not do anything).

I have removed the "use_ok" test from PNG.t.tmpl (this is deprecated).

I have moved libpng_doc to Libpng.pod.tmpl and set up the build
process to create "Libpng.pod" rather than including the POD into
Libpng.pm.