Do not modify interpolated expressions

[devmotion]

This PR contains the part of #64 that fixes #62.

[codecov]

Codecov Report

Merging #67 into master will decrease coverage by 1.71%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   79.42%   77.71%   -1.72%     
==========================================
  Files          13       13              
  Lines         841      839       -2     
==========================================
- Hits          668      652      -16     
- Misses        173      187      +14     

Impacted Files	Coverage Δ
src/utils.jl	53.06% <ø> (-3.55%)	⬇️
src/compiler.jl	79.86% <96.15%> (-9.40%)	⬇️
src/varname.jl	76.47% <100.00%> (+0.96%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9fbe09a...3a8f14a. Read the comment docs.

[yebai]

thanks @devmotion!