--exception:goto switch for deterministic exception handling (#12977)

This implements "deterministic" exception handling for Nim based on goto instead of setjmp. This means raising an exception is much cheaper than in C++'s table based implementations. Supports hard realtime systems. Default for --gc:arc and the C target because it's generally a good idea and arc is all about deterministic behavior.

Note: This implies that fatal runtime traps are not catchable anymore! This needs to be documented.

lib/system/refs_v2.nim

@@ -111,7 +111,7 @@ proc nimRawDispose(p: pointer) {.compilerRtl.} =
 template dispose*[T](x: owned(ref T)) = nimRawDispose(cast[pointer](x))
 #proc dispose*(x: pointer) = nimRawDispose(x)
 
-proc nimDestroyAndDispose(p: pointer) {.compilerRtl.} =
+proc nimDestroyAndDispose(p: pointer) {.compilerRtl, raises: [].} =
   let d = cast[ptr PNimType](p)[].destructor
   if d != nil: cast[DestructorProc](d)(p)
   when false: