_tools: extract most utility methods to a private package.

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

nmigen/hdl/ast.py

@@ -8,7 +8,7 @@ from collections.abc import Iterable, MutableMapping, MutableSet, MutableSequenc
 from enum import Enum
 
 from .. import tracer
-from ..tools import *
+from .._tools import *
 
 
 __all__ = [

nmigen/hdl/dsl.py

@@ -4,7 +4,7 @@ from contextlib import contextmanager
 from enum import Enum
 import warnings
 
-from ..tools import flatten, bits_for, deprecated
+from .._tools import flatten, bits_for, deprecated
 from .. import tracer
 from .ast import *
 from .ir import *

nmigen/hdl/ir.py

@@ -5,7 +5,7 @@ import warnings
 import traceback
 import sys
 
-from ..tools import *
+from .._tools import *
 from .ast import *
 from .cd import *
 

nmigen/hdl/rec.py

@@ -3,7 +3,7 @@ from collections import OrderedDict
 from functools import reduce
 
 from .. import tracer
-from ..tools import union, deprecated
+from .._tools import union, deprecated
 from .ast import *
 
 

nmigen/hdl/xfrm.py

@@ -2,7 +2,7 @@ from abc import ABCMeta, abstractmethod
 from collections import OrderedDict
 from collections.abc import Iterable
 
-from ..tools import flatten, deprecated
+from .._tools import flatten, deprecated
 from .. import tracer
 from .ast import *
 from .ast import _StatementList