Don't substitute in non-symbolic expressions

[hersle]

Adding this dispatch makes ModelingToolkit#2997 work on my machine.
I don't think it addresses the root cause of that issue, but at least it circumvents whatever triggers the bug.
Anyway, this PR sounds like a safe optimization to me. Let me know what you think.

[bowenszhu]

Thanks for the PR! Could you add some tests?

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Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 30.38%. Comparing base (a120309) to head (7ef8d1f).
Report is 182 commits behind head on master.

Files with missing lines	Patch %	Lines
src/variable.jl	0.00%	3 Missing ⚠️

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Flag	BASE (a120309)	HEAD (7ef8d1f)
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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1245       +/-   ##
===========================================
- Coverage   76.20%   30.38%   -45.83%     
===========================================
  Files          36       46       +10     
  Lines        3749     4572      +823     
===========================================
- Hits         2857     1389     -1468     
- Misses        892     3183     +2291     

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[hersle]

I'm gonna close this for now, because I think #1247 was a better way of fixing the issue.

Not substituting in non-symbolic expressions would also be breaking. For example, it would change the result of substitute(1, 1 => 2) from being 2 (which I would expect) to becoming 1 (which would surprise me). The same should be true for fast_substitute, for consistency. I feel like it's better to just leave things as they are.

Sorry for the bother :)