Allow ccalls that hardcode 2 Symbols

[timholy]

This was discovered in #13

[codecov-io]

Codecov Report

Merging #14 into master will increase coverage by 0.47%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   70.72%   71.19%   +0.47%     
==========================================
  Files           6        6              
  Lines        1117     1118       +1     
==========================================
+ Hits          790      796       +6     
+ Misses        327      322       -5

Impacted Files	Coverage Δ
src/interpret.jl	70.48% <100%> (+1.23%)	⬆️
src/builtins.jl	80% <0%> (+0.39%)	⬆️

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