_tools: extract most utility methods to a private package.

We don't want to guarantee backwards compatibility for most of them.

nmigen/compat/genlib/cdc.py

@@ -1,6 +1,6 @@
 import warnings
 
-from ...tools import deprecated
+from ..._tools import deprecated
 from ...lib.cdc import FFSynchronizer as NativeFFSynchronizer
 from ...hdl.ast import *
 from ..fhdl.module import CompatModule

nmigen/compat/genlib/fifo.py

@@ -1,4 +1,4 @@
-from ...tools import deprecated, extend
+from ..._tools import deprecated, extend
 from ...lib.fifo import (FIFOInterface as NativeFIFOInterface,
   SyncFIFO as NativeSyncFIFO, SyncFIFOBuffered as NativeSyncFIFOBuffered,
   AsyncFIFO as NativeAsyncFIFO, AsyncFIFOBuffered as NativeAsyncFIFOBuffered)

nmigen/compat/genlib/fsm.py

@@ -1,6 +1,6 @@
 from collections import OrderedDict
 
-from ...tools import _ignore_deprecated
+from ..._tools import _ignore_deprecated
 from ...hdl.xfrm import ValueTransformer, StatementTransformer
 from ...hdl.ast import *
 from ..fhdl.module import CompatModule, CompatFinalizeError

nmigen/compat/genlib/resetsync.py

@@ -1,4 +1,4 @@
-from ...tools import deprecated
+from ..._tools import deprecated
 from ...lib.cdc import ResetSynchronizer as NativeResetSynchronizer