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

lib/std/stackframes.nim

@@ -0,0 +1,30 @@
+const NimStackTrace = compileOption("stacktrace")
+const NimStackTraceMsgs = compileOption("stacktraceMsgs")
+
+template procName*(): string =
+  ## returns current C/C++ function name
+  when defined(c) or defined(cpp):
+    var name {.inject.}: cstring
+    {.emit: "`name` = __func__;".}
+    $name
+
+template getPFrame*(): PFrame =
+  ## avoids a function call (unlike `getFrame()`)
+  block:
+    when NimStackTrace:
+      var framePtr {.inject.}: PFrame
+      {.emit: "`framePtr` = &FR_;".}
+      framePtr
+
+template setFrameMsg*(msg: string, prefix = " ") =
+  ## attach a msg to current `PFrame`. This can be called multiple times
+  ## in a given PFrame. Noop unless passing --stacktraceMsgs and --stacktrace
+  when NimStackTrace and NimStackTraceMsgs:
+    block:
+      var fr {.inject.}: PFrame
+      {.emit: "`fr` = &FR_;".}
+      # consider setting a custom upper limit on size (analog to stack overflow)
+      frameMsgBuf.setLen fr.frameMsgLen
+      frameMsgBuf.add prefix
+      frameMsgBuf.add msg
+      fr.frameMsgLen += prefix.len + msg.len