_tools: extract most utility methods to a private package.

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

nmigen/compat/fhdl/bitcontainer.py

@@ -1,6 +1,6 @@
 from ... import tools
 from ...hdl import ast
-from ...tools import deprecated
+from ..._tools import deprecated
 
 
 __all__ = ["log2_int", "bits_for", "value_bits_sign"]

nmigen/compat/fhdl/module.py

@@ -1,6 +1,6 @@
 from collections.abc import Iterable
 
-from ...tools import flatten, deprecated
+from ..._tools import flatten, deprecated
 from ...hdl import dsl, ir
 
 
@@ -97,7 +97,7 @@ class _CompatModuleClockDomains(_CompatModuleProxy):
 class CompatModule(ir.Elaboratable):
     _Elaboratable__silence = True
 
-    # Actually returns nmigen.fhdl.Module, not a Fragment.
+    # Actually returns another nMigen Elaboratable (nmigen.dsl.Module), not a Fragment.
     def get_fragment(self):
         assert not self.get_fragment_called
         self.get_fragment_called = True

nmigen/compat/fhdl/specials.py

@@ -1,6 +1,6 @@
 import warnings
 
-from ...tools import deprecated, extend
+from ..._tools import deprecated, extend
 from ...hdl.ast import *
 from ...hdl.ir import Elaboratable
 from ...hdl.mem import Memory as NativeMemory

nmigen/compat/fhdl/structure.py

@@ -1,6 +1,6 @@
 from collections import OrderedDict
 
-from ...tools import deprecated, extend
+from ..._tools import deprecated, extend
 from ...hdl import ast
 from ...hdl.ast import (DUID, Value, Signal, Mux, Slice as _Slice, Cat, Repl, Const, C,
                         ClockSignal, ResetSignal,