Support immutables containing boxed values

[tkf]

No description provided.

[codecov]

Codecov Report

Merging #33 (0cc2059) into master (3b3f632) will decrease coverage by 0.98%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   81.45%   80.46%   -0.99%     
==========================================
  Files          12       12              
  Lines        1337     1341       +4     
==========================================
- Hits         1089     1079      -10     
- Misses        248      262      +14     

Flag	Coverage Δ
unittests	80.46% <100.00%> (-0.99%)	⬇️

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Impacted Files	Coverage Δ
src/workstealing.jl	92.30% <100.00%> (-0.62%)	⬇️
src/stack.jl	89.28% <0.00%> (-7.15%)	⬇️
src/lcrq.jl	76.16% <0.00%> (-4.07%)	⬇️
src/dlcrq.jl	74.06% <0.00%> (-1.16%)	⬇️
src/dict.jl	84.18% <0.00%> (-0.65%)	⬇️
src/msqueue.jl	85.41% <0.00%> (+6.25%)	⬆️

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