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/modulegraphs.nim

@@ -165,6 +165,7 @@ proc stopCompile*(g: ModuleGraph): bool {.inline.} =
 proc createMagic*(g: ModuleGraph; name: string, m: TMagic): PSym =
   result = newSym(skProc, getIdent(g.cache, name), nil, unknownLineInfo, {})
   result.magic = m
+  result.flags = {sfAlwaysReturn}
 
 proc newModuleGraph*(cache: IdentCache; config: ConfigRef): ModuleGraph =
   result = ModuleGraph()