Speed up Simulation by moving the declaration of the simple_func dict out of _execute

[fdxmw]

Before this change, pyrtlnet with pyrtl.Simulation runs in 2.2 seconds on my desktop, and after this change it runs in 1.3 seconds.

(pyrtlnet runs with pyrtl.FastSimulation by default, which runs in 0.1 seconds).

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.2%. Comparing base (0dbbbc6) to head (2b5fc5f).
⚠️ Report is 30 commits behind head on development.

Files with missing lines	Patch %	Lines
pyrtl/simulation.py	92.9%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           development    #493     +/-   ##
=============================================
- Coverage         92.2%   92.2%   -0.0%     
=============================================
  Files               30      30             
  Lines             7450    7452      +2     
=============================================
+ Hits              6865    6866      +1     
- Misses             585     586      +1     

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