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/hdl/__init__.py

@@ -0,0 +1,7 @@
+from .ast import Value, Const, C, Mux, Cat, Repl, Array, Signal, ClockSignal, ResetSignal
+from .dsl import Module
+from .cd import ClockDomain
+from .ir import Elaboratable, Fragment, Instance
+from .mem import Memory
+from .rec import Record
+from .xfrm import DomainRenamer, ResetInserter, CEInserter