Explicitly convert to Symbol when doing expression parsing

[Vaibhavdixit02]

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (43f93d0) 77.99% compared to head (07d4147) 17.60%.

Files	Patch %	Lines
src/parsing.jl	0.00%	2 Missing :warning:

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1017       +/-   ##
===========================================
- Coverage   77.99%   17.60%   -60.40%     
===========================================
  Files          28       28               
  Lines        3381     3335       -46     
===========================================
- Hits         2637      587     -2050     
- Misses        744     2748     +2004     

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[ChrisRackauckas]

Needs a test