fix wrong -Dp message cut

havent found the cause of the STR_LEN overflow but add some sanity check.
Add global convenience debugging macros
doc: vm doxygen docs
doc: gc.c
[GH #18] fix segv with test/closures/long.pn

and the wrong result in test/closures/upvals.pn.

horrible code duplication hack.
haven't found the probable cause - wrong stack ptr - yet.