Improve IAR construction

spot::iar() was fixed to handle correctly Rabin-like conditions.
It also now supports Streett-like conditions.

* NEWS, spot/twaalgos/postproc.cc: document it
* spot/twaalgos/rabin2parity.cc, spot/twaalgos/rabin2parity.hh:
  implement it
* tests/core/rabin2parity.test, tests/python/except.py: test it

tests/core/rabin2parity.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2017 Laboratoire de Recherche et
+# Copyright (C) 2017-2018 Laboratoire de Recherche et
 # Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -39,9 +39,39 @@ State: 1
 [0] 1 {1 }
 [!0] 1 {0 1 }
 --END--
+HOA: v1
+States: 1
+Start: 0
+AP: 1 "a"
+acc-name: Streett 1
+Acceptance: 2 Fin(0) | Inf(1)
+properties: trans-labels explicit-labels state-acc complete
+properties: deterministic
+--BODY--
+State: 0
+[t] 0
+--END--
+HOA: v1
+States: 3
+Start: 0
+AP: 1 "a"
+acc-name: Streett 3
+Acceptance: 6 (Fin(0) | Inf(1)) & (Fin(2) | Inf(3)) & (Fin(4) | Inf(5))
+properties: trans-labels explicit-labels trans-acc deterministic
+--BODY--
+State: 0
+[!0] 0 {1 3}
+[0] 1 {}
+State: 1
+[0] 2 {2}
+State: 2
+[!0] 0 {0 4}
+[0] 2 {}
+--END--
 EOF
 # random automata
-randaut 5 -n100 -u -D --acceptance="Rabin 0..6"
+randaut 4 -n100 -u -D --acceptance="Rabin 0..6"
+randaut 4 -n100 -u -D --acceptance="Streett 0..6"
 ) | \
 autcross \
   "autfilt --parity" \