[DON'T MERGE UNTIL DMD 2.111 IS RELEASED] remove dyaml.stdsumtype

[Herringway]

Use phobos's std.sumtype.

DMD 2.111 may possibly be released late this year. Opening early just to run the CI and make sure things haven't diverged too much.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.81%. Comparing base (8b18bb0) to head (e431706).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #330      +/-   ##
==========================================
+ Coverage   93.74%   93.81%   +0.07%     
==========================================
  Files          29       28       -1     
  Lines        4570     4126     -444     
==========================================
- Hits         4284     3871     -413     
+ Misses        286      255      -31     

Files	Coverage Δ
source/dyaml/node.d	91.06% <ø> (-0.80%)	⬇️

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