perf(util/list): use buffer of cells instead of buffer of T
perf(util/list): use buffer of cells instead of buffer of T

T may be a big object. We minimize the ammount of copying using buffer of (pointers to) cells.

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

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