better nativestacktrace support; refs #15284; backport [1.2] (#15384)

* nimStackTraceOverride: enable stack traces in exceptions

This is a two-step stack trace collection scheme, because re-raised
exceptions will collect multiple stack traces but use them rarely, when
printing info about an uncaught exception, so it makes sense to only do
the cheap stack unwinding all the time and the relatively expensive
debugging information collection on-demand.

`asyncfutures` implements its own `$` proc for printing
`seq[StackTraceEntry]`, so we have to add the debugging info there, just
like we do for the private `$` proc in `system/excpt`.

* cleaned up PR #15284

Co-authored-by: Ștefan Talpalaru <stefantalpalaru@yahoo.com>

lib/system/exceptions.nim

@@ -25,6 +25,11 @@ type
                             ## rendered at a later time, we should ensure the stacktrace
                             ## data isn't invalidated; any pointer into PFrame is
                             ## subject to being invalidated so shouldn't be stored.
+    when defined(nimStackTraceOverride):
+      programCounter*: uint ## Program counter - will be used to get the rest of the info,
+                            ## when `$` is called on this type. We can't use
+                            ## "cuintptr_t" in here.
+      procnameStr*, filenameStr*: string ## GC-ed objects holding the cstrings in "procname" and "filename"
 
   Exception* {.compilerproc, magic: "Exception".} = object of RootObj ## \
     ## Base exception class.