Update wip/verilator to version 4.016.
Major changes in version 4:
* Add multithreaded model generation.
* Add runtime arguments.
* Add GTKWave LXT2 native tracing.
* Add new reloop optimization for repetitive assignment compression.
* Add GTKWave FST native tracing.
* Support $past.
* Support restrict.
* Rename include/lxt2 to include/gtkwave.
* Support "ref" and "const ref" pins and functions.
* Removed --trace-lxt2, use --trace-fst instead.
* Support $fread.
* Add --trace-fst-thread.
* Support '#' comments in $readmem.
* Support "'dx" constants.
* For FST tracing use LZ4 compression.
* Support logical equivalence operator <->.
* Support deferred assertions.
* Add --quiet-exit.
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