u3: fix return type in hashtable trim when missing key#995
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This PR fixes a bug in hashtable trimming (introduced while refactoring in 98bcfaa). The wrong sentinel value was returned for a missing key in an intermediate node (
c3ninstead ofu3_none), making it appear incorrectly that a value had been trimmed and causing the hashtable entry-count to be wrongly decremented. This was effectively a space leak in the capped-cache variants of our HAMTs (ie, the persistent memoization cache), corrected only by melding.