_tools: extract most utility methods to a private package.

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

nmigen/back/pysim.py

@@ -6,7 +6,7 @@ from bitarray import bitarray
 from vcd import VCDWriter
 from vcd.gtkw import GTKWSave
 
-from ..tools import flatten
+from .._tools import flatten
 from ..hdl.ast import *
 from ..hdl.ir import *
 from ..hdl.xfrm import ValueVisitor, StatementVisitor

nmigen/back/rtlil.py

@@ -3,7 +3,7 @@ import textwrap
 from collections import defaultdict, OrderedDict
 from contextlib import contextmanager
 
-from ..tools import bits_for, flatten
+from .._tools import bits_for, flatten
 from ..hdl import ast, rec, ir, mem, xfrm