smtbmc: Add native json based witness format + smt2 backend support

This adds a native json based witness trace format. By having a common
format that includes everything we support, and providing a conversion
utility (yosys-witness) we no longer need to implement every format for
every tool that deals with witness traces, avoiding a quadratic
opportunity to introduce subtle bugs.

Included:

  * smt2: New yosys-smt2-witness info lines containing full hierarchical
    paths without lossy escaping.
  * yosys-smtbmc --dump-yw trace.yw: Dump results in the new format.
  * yosys-smtbmc --yw trace.yw: Read new format as constraints.
  * yosys-witness: New tool to convert witness formats.
    Currently this can only display traces in a human-readable-only
    format and do a passthrough read/write of the new format.
  * ywio.py: Small python lib for reading and writing the new format.
    Used by yosys-smtbmc and yosys-witness to avoid duplication.

.gitignore

@@ -28,6 +28,9 @@ __pycache__
 /yosys-smtbmc
 /yosys-smtbmc.exe
 /yosys-smtbmc-script.py
+/yosys-witness
+/yosys-witness.exe
+/yosys-witness-script.py
 /yosys-filterlib
 /yosys-filterlib.exe
 /kernel/*.pyh