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

compiler/ast.nim

@@ -229,7 +229,7 @@ type
   TNodeKinds* = set[TNodeKind]
 
 type
-  TSymFlag* = enum    # 41 flags!
+  TSymFlag* = enum    # 42 flags!
     sfUsed,           # read access of sym (for warnings) or simply used
     sfExported,       # symbol is exported from module
     sfFromGeneric,    # symbol is instantiation of a generic; this is needed
@@ -289,6 +289,8 @@ type
     sfTemplateParam   # symbol is a template parameter
     sfCursor          # variable/field is a cursor, see RFC 177 for details
     sfInjectDestructors # whether the proc needs the 'injectdestructors' transformation
+    sfAlwaysReturn    # proc can never raise an exception, not even OverflowError
+                      # or out-of-memory
 
   TSymFlags* = set[TSymFlag]