add subquery support as a distinct AST node#1

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DoctorVin wants to merge 4 commits intomasterfrom drvince/subqueries3

+239-3

Commits on Jun 25, 2020

add subquery support as a distinct AST node
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Commits on Jun 26, 2020

add a more-elaborate subquery test case
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another test case
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rustfmt
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