Remove nmigen.lib from prelude.

Currently it's just MultiReg, and there's no particularly good reason
to privilege this specific CDC primitive so much.

nmigen/lib/__init__.py

@@ -1 +0,0 @@
-from .cdc import MultiReg

nmigen/lib/fifo.py

@@ -4,6 +4,7 @@ from .. import *
 from ..asserts import *
 from ..tools import log2_int
 from .coding import GrayEncoder
+from .cdc import MultiReg
 
 
 __all__ = ["FIFOInterface", "SyncFIFO", "SyncFIFOBuffered", "AsyncFIFO", "AsyncFIFOBuffered"]