Support GC.at_preserve

[vchuravy]

No description provided.

[codecov-commenter]

Codecov Report

Merging #122 (c1b32b1) into master (9742b5f) will decrease coverage by 2.20%.
The diff coverage is 47.43%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
- Coverage   77.15%   74.95%   -2.21%     
==========================================
  Files          12       12              
  Lines        1922     2076     +154     
==========================================
+ Hits         1483     1556      +73     
- Misses        439      520      +81     

Impacted Files	Coverage Δ
src/api.jl	37.50% <0.00%> (-0.88%)	⬇️
src/compiler.jl	77.54% <46.32%> (-3.70%)	⬇️
src/compiler/validation.jl	63.44% <61.11%> (-0.20%)	⬇️

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[wsmoses]

@vchuravy rebased, this now passes

[vchuravy]

I am going to split the MPI changes into their own PR