_tools: extract most utility methods to a private package.

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

nmigen/test/compat/support.py

@@ -1,4 +1,4 @@
-from ...tools import _ignore_deprecated
+from ..._tools import _ignore_deprecated
 from ...compat import *
 from ...compat.fhdl import verilog
 

nmigen/test/compat/test_size.py

@@ -1,6 +1,6 @@
 import unittest
 
-from ...tools import _ignore_deprecated
+from ..._tools import _ignore_deprecated
 from ...compat import *
 
 

nmigen/test/test_sim.py

@@ -1,7 +1,7 @@
 from contextlib import contextmanager
 
 from .tools import *
-from ..tools import flatten, union
+from .._tools import flatten, union
 from ..hdl.ast import *
 from ..hdl.cd import  *
 from ..hdl.mem import *