Parameter of h_do_parse not on stack

Packrat avoids calling perform_lowlevel_parse if the result is already in the cache table. The script uses breakpoints at h_do_parse when stepping, meaning the "current" parser might never be pushed on the stack, if a different code path is taken. This is probably correct, but the presentation needs to be clarified.

[ ] Add command for displaying the parameter of h_do_parse as with backtrace [ ] Fix GUI widget to display the correct parser [ ] Maybe extend hammer-parser-backtrace to display the current parser separately from the stack