--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/tgotoexceptions2.nim

@@ -0,0 +1,104 @@
+discard """
+  cmd: "nim c --gc:arc --exceptions:goto $file"
+  output: '''
+B1
+B2
+catch
+A1
+1
+B1
+B2
+catch
+A1
+A2
+0
+B1
+B2
+A1
+1
+B1
+B2
+A1
+A2
+3
+A
+B
+C
+'''
+"""
+
+# More thorough test of return-in-finaly
+
+var raiseEx = true
+var returnA = true
+var returnB = false
+
+proc main: int =
+  try: #A
+    try: #B
+      if raiseEx:
+        raise newException(OSError, "")
+      return 3
+    finally: #B
+      echo "B1"
+      if returnB:
+        return 2
+      echo "B2"
+  except OSError: #A
+    echo "catch"
+  finally: #A
+    echo "A1"
+    if returnA:
+      return 1
+    echo "A2"
+
+for x in [true, false]:
+  for y in [true, false]:
+    # echo "raiseEx: " & $x
+    # echo "returnA: " & $y
+    # echo "returnB: " & $z
+    # in the original test returnB was set to true too and
+    # this leads to swallowing the OSError exception. This is
+    # somewhat compatible with Python but it's non-sense, 'finally'
+    # should not be allowed to swallow exceptions. The goto based
+    # implementation does something sane so we don't "correct" its
+    # behavior just to be compatible with v1.
+    raiseEx = x
+    returnA = y
+    echo main()
+
+# Various tests of return nested in double try/except statements
+
+proc test1() =
+
+  defer: echo "A"
+
+  try:
+    raise newException(OSError, "Problem")
+  except OSError:
+    return
+
+test1()
+
+
+proc test2() =
+
+  defer: echo "B"
+
+  try:
+    return
+  except OSError:
+    discard
+
+test2()
+
+proc test3() =
+  try:
+    try:
+      raise newException(OSError, "Problem")
+    except OSError:
+      return
+  finally:
+    echo "C"
+
+test3()