(Tagging upstream because the Yosys cxxrtl backend needs to be taught about asserts, too.)

This issue was discussed on August 16th as part of the weekly IRC meeting: https://freenode.irclog.whitequark.org/nmigen/2020-08-17#1597691839-1597695703;

In short:

• We should use AssertionError (or an exception inherited from AssertionError if needed)
• Exceptions should not be thrown deep inside generated pysim code. We should instead generate events that will be handled so that we don't break the VCD generation
• We should append to tracebacks to make errors more legible (polyfill for Python 3.6: https://github.com/pallets/jinja/blob/c074fd5ecb7af70b2f864c9ef545fffd38dc203b/src/jinja2/debug.py)
• Assume should do the same things as Assert

Support for capturing assertions in CXXRTL is about to land upstream as part of YosysHQ/yosys#4128.

Amaranth will require modification to emit $check cell from Assert statements once that PR lands. The format string within $check cell is the same as one within $print cell, which will be the basis of the implementation of #432.

Support for this feature has been accepted via RFC 50 and implementation is tracked in issue #1186.