lib.fifo: make simulation read() and write() functions compat-only.

These functions were originally changed in 3ed51938, in an attempt
to make them take one cycle instead of two. However, this does not
actually work because of drawbacks of the simulator interface.

Avoid committing to any specific implementation for now, and instead
make them compat-only extensions.

nmigen/test/test_lib_fifo.py

@@ -5,42 +5,6 @@ from ..back.pysim import *
 from ..lib.fifo import *
 
 
-class FIFOSmokeTestCase(FHDLTestCase):
-    def assertSyncFIFOWorks(self, fifo, xfrm=lambda x: x):
-        with Simulator(xfrm(Fragment.get(fifo, None)), vcd_file=open("test.vcd", "w")) as sim:
-            sim.add_clock(1e-6)
-            def process():
-                yield from fifo.write(1)
-                yield from fifo.write(2)
-                while not (yield fifo.r_rdy):
-                    yield
-                if not fifo.fwft:
-                    yield fifo.r_en.eq(1)
-                yield
-                self.assertEqual((yield from fifo.read()), 1)
-                self.assertEqual((yield from fifo.read()), 2)
-            sim.add_sync_process(process)
-            sim.run()
-
-    def assertAsyncFIFOWorks(self, fifo):
-        self.assertSyncFIFOWorks(fifo, xfrm=DomainRenamer({"read": "sync", "write": "sync"}))
-
-    def test_sync_fwft(self):
-        self.assertSyncFIFOWorks(SyncFIFO(width=8, depth=4, fwft=True))
-
-    def test_sync_not_fwft(self):
-        self.assertSyncFIFOWorks(SyncFIFO(width=8, depth=4, fwft=False))
-
-    def test_sync_buffered(self):
-        self.assertSyncFIFOWorks(SyncFIFOBuffered(width=8, depth=4))
-
-    def test_async(self):
-        self.assertAsyncFIFOWorks(AsyncFIFO(width=8, depth=4))
-
-    def test_async_buffered(self):
-        self.assertAsyncFIFOWorks(AsyncFIFOBuffered(width=8, depth=3))
-
-
 class FIFOModel(Elaboratable, FIFOInterface):
     """
     Non-synthesizable first-in first-out queue, implemented naively as a chain of registers.