- Add script to drop parameters from VCDs.
- Bump the version of iverilog used in CI. I couldn't use anything never
  than eab37efb due a pending iverilog issue.
- Standardize around Python 3 in test cases.

- swap 1364-2001 and 1364-2001-noconfig
- fix existing begin_keywords test
- add keyword test suite for all keywords

- The deleted chunk in TFBlock has been unreachable since 57ef23ef. The
  RemoveComments change would make it reachable by dropping the leading
  Null in a task or function body.
- The optimization in TFBlock was unsound because the nested block could
  shadow identifiers used (but not defined) in the tail of the outer
  block.
- The optimization is not clearly necessary, so I'm dropping it for now.
- Expand tf_block test coverage.

- test deprecated flags
- test struct const edge cases
- test implicit net edge cases
- test dump-prefix

These are now kept while still supporting optional trailing commas and
without introducing pass-through inconsistencies.

- fold string literal comparisons
- fold non-decimal bit shifts
- fold non-decimal integer comparisons
- fold decimal bitwise AND and OR
- simplify cast expressions before elaboration
- remove duplicate cast expression traversal
- flatten concatenated numbers in a single pass

The implicit type of a localparam is not always needed. This improves
performance when complex localparam expressions are processed.

- event expressions use Expr over LHS
- stricter AST representation of event controls
- property specs use event expressions directly