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.

examples/basic/cdc.py

@@ -1,4 +1,5 @@
 from nmigen import *
+from nmigen.lib.cdc import MultiReg
 from nmigen.cli import main
 
 

nmigen/__init__.py

@@ -5,4 +5,3 @@ except pkg_resources.DistributionNotFound:
     pass
 
 from .hdl import *
-from .lib import *

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"]