--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.

tests/destructor/tgotoexceptions.nim

@@ -0,0 +1,117 @@
+discard """
+  output: '''
+msg1
+msg2
+finally2
+finally1
+begin
+one iteration!
+caught!
+except1
+finally1
+caught! 2
+BEFORE
+FINALLY
+BEFORE
+EXCEPT
+FINALLY
+RECOVER
+BEFORE
+EXCEPT: IOError: hi
+FINALLY
+'''
+  cmd: "nim c --gc:arc --exceptions:goto $file"
+"""
+
+#bug 7204
+proc nested_finally =
+  try:
+    raise newException(KeyError, "msg1")
+  except KeyError as ex:
+    echo ex.msg
+    try:
+      raise newException(ValueError, "msg2")
+    except:
+      echo getCurrentExceptionMsg()
+    finally:
+      echo "finally2"
+  finally:
+    echo "finally1"
+
+nested_finally()
+
+proc doraise =
+  raise newException(ValueError, "gah")
+
+proc main =
+  while true:
+    try:
+      echo "begin"
+      doraise()
+    finally:
+      echo "one ", "iteration!"
+
+try:
+  main()
+except:
+  echo "caught!"
+
+when true:
+  proc p =
+    try:
+      raise newException(Exception, "Hello")
+    except:
+      echo "except1"
+      raise
+    finally:
+      echo "finally1"
+
+  try:
+    p()
+  except:
+    echo "caught! 2"
+
+
+proc noException =
+  try:
+    echo "BEFORE"
+
+  except:
+    echo "EXCEPT"
+    raise
+
+  finally:
+    echo "FINALLY"
+
+try: noException()
+except: echo "RECOVER"
+
+proc reraise_in_except =
+  try:
+    echo "BEFORE"
+    raise newException(IOError, "")
+
+  except IOError:
+    echo "EXCEPT"
+    raise
+
+  finally:
+    echo "FINALLY"
+
+try: reraise_in_except()
+except: echo "RECOVER"
+
+proc return_in_except =
+  try:
+    echo "BEFORE"
+    raise newException(IOError, "hi")
+
+  except:
+    echo "EXCEPT: ", getCurrentException().name, ": ", getCurrentExceptionMsg()
+    return
+
+  finally:
+    echo "FINALLY"
+
+try: return_in_except()
+except: echo "RECOVER"