PyMTL 4.0 release staging

[ptpan]

This PR includes changes that will be part of the PyMTL 4.0 release. Here is a list of changes included in this PR (WIP):

• Support for Verilator 5.016, including Verilog import support and CI setup
• Revamped Verilog import support to address data corruption / segfaults
• Support for parallel pytest runs with the xdist module
• Revamp stdlib; this may break existing code that uses pymtl3.stdlib
• Support for specification-based testing

[codecov]

Codecov Report

Attention: 179 lines in your changes are missing coverage. Please review.

Comparison is base (fc7c3fa) 89.18% compared to head (6da7397) 90.27%.
Report is 2 commits behind head on master.

❗ Current head 6da7397 differs from pull request most recent head 8084e12. Consider uploading reports for the commit 8084e12 to get more accurate results

Files	Patch %	Lines
pymtl3/stdlib/mem/MemRequesterAdapterFL.py	20.63%	50 Missing ⚠️
pymtl3/stdlib/xcel/XcelRequesterAdapterFL.py	19.60%	41 Missing ⚠️
pymtl3/stdlib/stream/OStreamEnqAdapterFL.py	28.00%	18 Missing ⚠️
pymtl3/stdlib/stream/IStreamDeqAdapterFL.py	35.00%	13 Missing ⚠️
...ends/verilog/import_/VerilogVerilatorImportPass.py	85.29%	10 Missing ⚠️
...sses/rtlir/structural/StructuralRTLIRSignalExpr.py	75.00%	6 Missing ⚠️
pymtl3/stdlib/test_utils/test_helpers.py	50.00%	5 Missing ⚠️
.../passes/backends/verilog/VerilogPlaceholderPass.py	0.00%	4 Missing ⚠️
pymtl3/stdlib/mem/MemoryFL.py	87.87%	4 Missing ⚠️
pymtl3/stdlib/stream/ifcs/ifcs.py	66.66%	4 Missing ⚠️
... and 13 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   89.18%   90.27%   +1.08%     
==========================================
  Files         330      317      -13     
  Lines       30440    29199    -1241     
==========================================
- Hits        27147    26358     -789     
+ Misses       3293     2841     -452     

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[ptpan]

Closing this to rename the dev branch to pymtl4.0-dev