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/vendor/fpga/lattice_ice40.py

@@ -3,9 +3,7 @@ import os
 import subprocess
 import tempfile
 
-from ...hdl.ast import *
-from ...hdl.dsl import *
-from ...hdl.ir import *
+from ...hdl import *
 from ...build import *