Allow setting a default Symbol or String

[lrennels]

Handles #873 and allows us to set Symbols or Strings as defaults.

[codecov]

Codecov Report

Merging #874 (943fd46) into master (5c683a5) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #874      +/-   ##
==========================================
- Coverage   84.34%   84.28%   -0.06%     
==========================================
  Files          40       40              
  Lines        3794     3799       +5     
==========================================
+ Hits         3200     3202       +2     
- Misses        594      597       +3     

Flag	Coverage Δ
unittests	84.28% <100.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/defcomp.jl	79.03% <100.00%> (-1.08%)	⬇️

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