Demangles bt, possibly got from debug_get_raw_backtrace, by
replacing addresses with actual function calls and eventually line numbers, if available.
It modifies lines of bt, but skips those it cannot parse.
Note: Getting backtraces only works if the library was configured with --enable-backtraces.
See also debug_get_raw_backtrace
| bt |
a StringBuilder with a raw backtrace, or null |