Work toward reducing the incremental size of an ExprList object to 24-byte

Work toward reducing the incremental size of an ExprList object to 24-byte
per entry, from 32-bytes (on a 64-bit machine).  This helps the new
mini-lookaside allocator to run better by avoiding excessive reallocs.
The current change mostly works, but still has a few loose ends to tie up.
This check-in is merely a snapshot to save my work.

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