hdl.{dsl,mem,xfrm}: inject appropriate source locations.

hdl.{dsl,mem,xfrm}: inject appropriate source locations.

This primarily fixes the problem with source location precision in
Module (which used to trace locations from __exit__ of the context
managers, by which point everything interesting has been lost), but
also improves memory port and control inserter source locations.

On the sample of examples/basic/*.py, the only incorrectly inferred
remaining location is clk pointing to hdl/mem.py:166.