Unmarshal subproblems from problem details#138

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mbaynton wants to merge 2 commits intogolang:masterfrom mbaynton:subproblems

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Commits on May 10, 2020

Unmarshal subproblems and titles from problem details
mbaynton
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Commits on May 11, 2020

Changes per Roland's excellent code review
mbaynton
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