implement stack trace introspection

[Araq]

Probably implement it on top of https://github.com/gcc-mirror/gcc/blob/master/libbacktrace/backtrace.h

It should be its own module, std / stacktraces, not part of system.nim:

API:

# stacktraces.nim

proc writeStackTrace(f: var File)
proc writeStackTrace() # stdout or stderr, I don't remember what Nim uses

proc getStackTrace(): string

The feature should demangle the function names.