Fix GC Preserve

[wsmoses]

Fixes #125

[codecov-commenter]

Codecov Report

Merging #133 (90309a3) into master (d9113ac) will increase coverage by 0.04%.
The diff coverage is 63.84%.

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
+ Coverage   77.03%   77.08%   +0.04%     
==========================================
  Files          12       12              
  Lines        1999     2007       +8     
==========================================
+ Hits         1540     1547       +7     
- Misses        459      460       +1     

Impacted Files	Coverage Δ
src/compiler.jl	81.40% <63.28%> (+0.19%)	⬆️
src/compiler/validation.jl	63.75% <100.00%> (+0.30%)	⬆️
src/api.jl	35.63% <0.00%> (-1.87%)	⬇️
src/typetree.jl	81.11% <0.00%> (-1.12%)	⬇️

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