sim: use Simulator as a contextmanager

__del__ garbage collector callbacks are too delicate.  E.g. imported
modules can be garbage collected before the objects using them. Can't
use os.remove, socket.SHUT_RDWR...

* added a DeprecationWarning if a Simulator is garbage collected without
having its .close() called
* renamed all gc __del__ callbacks to close()
* implemented context manager hooks for Simulator. Use like

   with Simulator(TestBench()) as s:
       s.run()

migen/sim/generic.py

@@ -1,3 +1,5 @@
+import warnings
+
 from migen.fhdl.std import *
 from migen.fhdl.structure import _Fragment
 from migen.fhdl import verilog
@@ -177,9 +179,23 @@ class Simulator:
 				self.multiwrite(getattr(obj, k), v)
 
 	def __del__(self):
+		if hasattr(self, "ipc"):
+			warnings.warn("call Simulator.close() to clean up "
+					"or use it as a contextmanager", DeprecationWarning)
+			self.close()
+
+	def close(self):
+		self.ipc.close()
+		self.sim_runner.close()
 		del self.ipc
 		del self.sim_runner
 
+	def __enter__(self):
+		return self
+
+	def __exit__(self,  type, value, traceback):
+		self.close()
+
 # Contrary to multiread/multiwrite, Proxy fetches the necessary signals only and
 # immediately forwards writes into the simulation.
 class Proxy:

migen/sim/icarus.py

@@ -3,6 +3,7 @@
 
 import subprocess
 import os
+import time
 
 def _str2file(filename, contents):
 	f = open(filename, "w")
@@ -27,8 +28,12 @@ class Runner:
 		subprocess.check_call(["iverilog", "-o", self.vvp_file] + self.options + [self.top_file, self.dut_file] + self.extra_files)
 		self.process = subprocess.Popen(["vvp", "-mmigensim", self.vvp_file])
 
-	def __del__(self):
+	def close(self):
 		if hasattr(self, "process"):
+			self.process.terminate()
+			if self.process.poll() is None:
+				time.sleep(.1)
+				self.process.kill()
 			self.process.wait()
 		if not self.keep_files:
 			for f in [self.top_file, self.dut_file, self.vvp_file]:

migen/sim/ipc.py

@@ -166,7 +166,7 @@ class Initiator:
 			raise PacketTooLarge
 		return _unpack(packet)
 
-	def __del__(self):
+	def close(self):
 		if hasattr(self, "conn"):
 			self.conn.shutdown(socket.SHUT_RDWR)
 			self.conn.close()