Support Missing in union attribute types

[slwu89]

Mainly, adds support for attribute types which are union types including Missing. This entails adding specialized methods for preimage to deal with this case, as the usual == comparison doesn't work with the singleton object missing.

The cursed case where an attribute type is something like Union{Nothing,Missing,...} will still not work, but I think that is ok.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (9d9c019) 89.61% compared to head (07fb97d) 89.93%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   89.61%   89.93%   +0.32%     
==========================================
  Files          12       12              
  Lines        1078     1083       +5     
==========================================
+ Hits          966      974       +8     
+ Misses        112      109       -3     

Files	Coverage Δ
src/ColumnImplementations.jl	91.11% <100.00%> (+0.20%)	⬆️
src/PreimageCaches.jl	78.43% <100.00%> (+1.83%)	⬆️

... and 1 file with indirect coverage changes

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[epatters]

Thanks Sean! One small comment to simplify the logic.

[epatters]

Thanks Sean, looks great!