Implement fchmod, rename(at) sandboxing for compilers

[quantum5]

No description provided.

[codecov-commenter]

Codecov Report

Merging #933 (c554fca) into master (423afd5) will increase coverage by 0.17%.
The diff coverage is 90.74%.

@@            Coverage Diff             @@
##           master     #933      +/-   ##
==========================================
+ Coverage   86.52%   86.70%   +0.17%     
==========================================
  Files         138      138              
  Lines        4676     4791     +115     
==========================================
+ Hits         4046     4154     +108     
- Misses        630      637       +7     

Impacted Files	Coverage Δ
dmoj/executors/compiled_executor.py	91.86% <86.48%> (-0.90%)	⬇️
dmoj/cptbox/isolate.py	95.79% <100.00%> (-0.04%)	⬇️
dmoj/utils/helper_files.py	95.29% <0.00%> (+4.81%)	⬆️

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[quantum5]

retest this please

[quantum5]

retest this please