rewritten goto based exception handling; much cleaner implementation;… (#13677)

* rewritten goto based exception handling; much cleaner implementation; fixes #13668

tests/destructor/tgotoexceptions8.nim

@@ -0,0 +1,69 @@
+discard """
+  output: '''A
+B
+X
+inner finally
+Y
+outer finally
+msg1
+msg2
+finally2
+finally1'''
+  cmd: "nim c --gc:arc $file"
+"""
+
+# bug #13668
+
+proc main =
+  try:
+    try:
+      raise newException(IOError, "IOError")
+
+    except:
+      echo "A"
+      raise newException(CatchableError, "CatchableError")
+
+  except:
+    echo "B"
+    #discard
+
+proc mainB =
+  try:
+    try:
+      raise newException(IOError, "IOError")
+
+    except:
+      echo "X"
+      raise newException(CatchableError, "CatchableError")
+    finally:
+      echo "inner finally"
+
+  except:
+    echo "Y"
+    #discard
+  finally:
+    echo "outer finally"
+
+main()
+mainB()
+
+when true:
+  #bug 7204
+  proc nested_finally =
+    try:
+      raise newException(KeyError, "msg1")
+    except KeyError as ex:
+      echo ex.msg
+      try:
+        # pop exception
+        raise newException(ValueError, "msg2") # push: exception stack (1 entry)
+      except:
+        echo getCurrentExceptionMsg()
+        # pop exception (except)
+      finally:
+        echo "finally2"
+      # pop exception (except KeyError as ex)
+    finally:
+      echo "finally1"
+
+  nested_finally()