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

changelog.md

@@ -161,6 +161,14 @@ echo f
 - The compiler now inferes "sink parameters". To disable this for a specific routine,
   annotate it with `.nosinks`. To disable it for a section of code, use
   `{.push sinkInference: off.}`...`{.pop.}`.
+- The compiler now supports a new switch `--panics:on` that turns runtime
+  errors like `IndexError` or `OverflowError` into fatal errors that **cannot**
+  be caught via Nim's `try` statement. `--panics:on` can improve the
+  runtime efficiency and code size of your program significantly.
+- The compiler now warns about inheriting directly from `system.Exception` as
+  this is **very bad** style. You should inherit from `ValueError`, `IOError`,
+  `OSError` or from a different specific exception type that inherits from
+  `CatchableError` and cannot be confused with a `Defect`.
 
 
 ## Bugfixes