Implement RFC 66: Simulation time.

[zyp]

No description provided.

[whitequark]

The implementation looks pretty good--can we have tests?

[whitequark]

You should add a changelog entry.

Looks good to me on the first pass.

@wanda-phi, could you take a look as well?

[codecov]

Codecov Report

Attention: Patch coverage is 92.01681% with 19 lines in your changes missing coverage. Please review.

Project coverage is 91.25%. Comparing base (74a4096) to head (54ed9d1).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
amaranth/build/plat.py	33.33%	6 Missing ⚠️
amaranth/build/dsl.py	77.77%	3 Missing and 1 partial ⚠️
amaranth/sim/core.py	78.57%	2 Missing and 1 partial ⚠️
amaranth/build/res.py	83.33%	1 Missing and 1 partial ⚠️
amaranth/hdl/_time.py	98.83%	0 Missing and 2 partials ⚠️
amaranth/sim/_async.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1461      +/-   ##
==========================================
+ Coverage   91.15%   91.25%   +0.10%     
==========================================
  Files          43       44       +1     
  Lines       11153    11446     +293     
  Branches     2715     2800      +85     
==========================================
+ Hits        10166    10445     +279     
- Misses        818      827       +9     
- Partials      169      174       +5     

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[whitequark]

LGTM