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/compat/genlib/fifo.py

@@ -1,4 +1,4 @@
-from ...tools import deprecated
+from ...tools import deprecated, extend
 from ...lib.fifo import FIFOInterface as NativeFIFOInterface, \
   SyncFIFO, SyncFIFOBuffered, AsyncFIFO, AsyncFIFOBuffered
 
@@ -14,3 +14,25 @@ class CompatFIFOInterface(NativeFIFOInterface):
 
 
 _FIFOInterface = CompatFIFOInterface
+
+
+@extend(NativeFIFOInterface)
+def read(self):
+    """Read method for simulation."""
+    assert (yield self.r_rdy)
+    value = (yield self.r_data)
+    yield self.r_en.eq(1)
+    yield
+    yield self.r_en.eq(0)
+    yield
+    return value
+
+@extend(NativeFIFOInterface)
+def write(self, data):
+    """Write method for simulation."""
+    assert (yield self.w_rdy)
+    yield self.w_data.eq(data)
+    yield self.w_en.eq(1)
+    yield
+    yield self.w_en.eq(0)
+    yield