migen/genlib/record: add leave_out parameter to connect

Modules doing dataflow adaptation often need to connect most of the signals between endpoints except the one concerned by the adaptation.
This new parameter ease that by avoid manual connection of all signals.

migen/genlib/record.py

@@ -128,10 +128,13 @@ class Record:
     def raw_bits(self):
         return Cat(*self.flatten())
 
-    def connect(self, *slaves):
+    def connect(self, *slaves, leave_out=set()):
+        if isinstance(leave_out, str):
+            leave_out = {leave_out}
         r = []
         for f in self.layout:
             field = f[0]
+            if field not in leave_out:
                 self_e = getattr(self, field)
                 if isinstance(self_e, Signal):
                     direction = f[2]
@@ -143,7 +146,7 @@ class Record:
                         raise TypeError
                 else:
                     for slave in slaves:
-                    r += self_e.connect(getattr(slave, field))
+                        r += self_e.connect(getattr(slave, field), leave_out=leave_out)
         return r
 
     def connect_flat(self, *slaves):