stacktraces can now show custom runtime msgs per frame (#13351)

* stacktraces can now show custom runtime msgs
* improve tests/stdlib/tstackframes.nim
* fix test for --gc:arc
* test --stacktraceMsgs:on and --stacktraceMsgs:off
* --stacktracemsgs:off by default

tests/stdlib/mstackframes.nim

@@ -0,0 +1,38 @@
+import std/stackframes
+
+
+
+# line 5
+var count = 0
+
+proc main1(n: int) =
+  setFrameMsg $("main1", n)
+  if n > 0:
+    main1(n-1)
+
+proc main2(n: int) =
+  count.inc
+  setFrameMsg $("main2", n, count)
+  proc bar() =
+    setFrameMsg $("bar ",)
+    if n < 3: raise newException(CatchableError, "on purpose")
+  bar()
+  main2(n-1)
+
+proc main() =
+  var z = 0
+  setFrameMsg "\n  z: " & $z, prefix = ""
+  # multiple calls inside a frame are possible
+  z.inc
+  setFrameMsg "\n  z: " & $z, prefix = ""
+  try:
+    main2(5)
+  except CatchableError:
+    main1(10) # goes deep and then unwinds; sanity check to ensure `setFrameMsg` from inside
+              # `main1` won't invalidate the stacktrace; if StackTraceEntry.frameMsg
+              # were a reference instead of a copy, this would fail.
+    let e = getCurrentException()
+    let trace = e.getStackTrace
+    echo trace
+
+main()