Fix small memory leak and validate package.json with CI cache

[texodus]

Fix a few small memory leaks which in aggregate caused OOM and other test stability issues, as per failing master build. This was not detected in CI as the emsdk cache was keyed on yarn.lock, while emsdk versions are not yarn dependencies.

One of these memory leaks was a string-length miscalculation in the join aggregate, as well as an assumption that these values already be strings; the fix for this behavior also fixes #1786