Add the ability to create "canonical symbolic contexts"

[daemontus]

This is rather intuitive: We can now convert an existing SymbolicContext with a bunch of extra variables into the "canonical" context which has nothing except for the state/parameter variables.

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (c5ebee8) 80.44% compared to head (ab55033) 80.47%.

Files	Patch %	Lines
src/symbolic_async_graph/_impl_symbolic_context.rs	82.75%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   80.44%   80.47%   +0.02%     
==========================================
  Files          72       72              
  Lines        4890     4948      +58     
==========================================
+ Hits         3934     3982      +48     
- Misses        956      966      +10     

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