Clean up imports.

This commit:
  * moves lists of universally useful imports from `nmigen` to
    `nmigen.hdl` and `nmigen.lib`, reimporting them in `nmigen`;
  * replaces lots of imports from individual parts of `nmigen.hdl`
    with a star import from `nmigen.hdl`;
  * replaces imports in tests with what we expect downstream code
    to use;
  * adds some missing imports in `nmigen.formal`.

nmigen/test/test_lib_cdc.py

@@ -1,7 +1,5 @@
 from .tools import *
-from ..hdl.ast import *
-from ..hdl.cd import *
-from ..hdl.dsl import *
+from ..hdl import *
 from ..back.pysim import *
 from ..lib.cdc import *
 

nmigen/test/test_lib_coding.py

@@ -1,7 +1,6 @@
 from .tools import *
-from ..hdl.ast import *
-from ..hdl.dsl import *
-from ..hdl.ir import *
+from ..hdl import *
+from ..formal import *
 from ..back.pysim import *
 from ..lib.coding import *
 

nmigen/test/test_lib_fifo.py

@@ -1,10 +1,6 @@
 from .tools import *
-from ..hdl.ast import *
-from ..hdl.dsl import *
-from ..hdl.mem import *
-from ..hdl.ir import *
-from ..hdl.xfrm import *
-from ..hdl.cd import *
+from ..hdl import *
+from ..formal import *
 from ..back.pysim import *
 from ..lib.fifo import *
 

nmigen/test/test_lib_io.py

@@ -1,6 +1,7 @@
 from .tools import *
-from ..hdl.ast import *
+from ..hdl import *
 from ..hdl.rec import *
+from ..back.pysim import *
 from ..lib.io import *