catchable defects (#13626)

* allow defects to be caught even for --exceptions:goto (WIP)
* implemented the new --panics:on|off switch; refs https://github.com/nim-lang/RFCs/issues/180
* new implementation for integer overflow checking
* produce a warning if a user-defined exception type inherits from Exception directly
* applied Timothee's suggestions; improved the documentation and replace the term 'checked runtime check' by 'panic'
* fixes #13627
* don't inherit from Exception directly

tests/destructor/tgotoexceptions7.nim

@@ -0,0 +1,52 @@
+discard """
+  cmd: "nim c --gc:arc --exceptions:goto --panics:off $file"
+  output: '''field error prevented
+prevented!
+caught
+AssertionError
+900'''
+"""
+
+type
+  E = enum
+    kindA, kindB
+  Obj = object
+    case kind: E
+    of kindA: s: string
+    of kindB: i: int
+
+  ObjA = ref object of RootObj
+  ObjB = ref object of ObjA
+
+proc takeRange(x: range[0..4]) = discard
+
+proc bplease(x: ObjB) = discard
+
+proc helper = doAssert(false)
+
+proc main(i: int) =
+  var obj = Obj(kind: kindA, s: "abc")
+  try:
+    obj.kind = kindB
+  except FieldError:
+    echo "field error prevented"
+
+  try:
+    var objA = ObjA()
+    bplease(ObjB(objA))
+  except ObjectConversionError:
+    echo "prevented!"
+
+  try:
+    takeRange(i)
+  except RangeError:
+    echo "caught"
+
+  try:
+    helper()
+  except AssertionError:
+    echo "AssertionError"
+
+  echo i * i
+
+main(30)