Add stacktrace decoder

Usage:

    $ nm mini-os | sort > symbols
    # copy/paste to Mini/OS's call stack to callstack.txt
    $ ./scripts/decode_callstack symbols callstack.txt
Remove junk file
Update stub generator: support char array
Rewrite xencons_ring_init
Bugfix: do not increase megablock counte, when re-use megablock from free list