Allow visualizing delta cycles in VCD dumps

[whitequark]

This commit adds an option fs_per_delta= to Simulator.write_vcd(). Specifying a positive integer value for it causes the simulator to offset value change times by that many femtoseconds for each delta cycle after the last timeline advancement.

This option is only suitable for debugging. If the timeline is advanced by less than the combined duration of expanded delta cycles, an error will be raised like:

vcd.writer.VCDPhaseError: Out of order timestamp: 62490

Typically fs_per_delta=1 is best, since it allows thousands of delta cycles to be expanded without risking a VCD phase error, but bigger values can be used for an exaggerated visual effect.

Also, the VCD writer is changed to use 1 fs as the timebase instead of 1 ps. This change is largely invisible to designers, resulting only in slightly larger VCD files due to longer timestamps.

Since the fs_per_delta= option is per VCD writer, it is possible to simultaneously dump two VCDs, one with and one without delta cycle expansion:

with sim.write_vcd("sim.vcd"), sim.write_vcd("dsim.vcd", fs_per_delta=1):
    sim.run()

In GTKWave, it is possible to observe every intermediate value recorded by the VCD writer by selecting all signals and using left and right arrow keys to navigate. This works regardless of the value of fs_per_delta.

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• Depends on Only preempt simulator testbenches on explicit wait points #1231

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.81%. Comparing base (0cb71f8) to head (4824a6a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1232      +/-   ##
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+ Coverage   89.80%   89.81%   +0.01%     
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  Files          43       43              
  Lines        9923     9928       +5     
  Branches     2400     2399       -1     
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+ Hits         8911     8917       +6     
+ Misses        821      820       -1     
  Partials      191      191              

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