(per Monty code review)
The notion of storing LLMemoryInfo data both as an LLSD::Map and an
LLSD::Array of pair arrays arose from a (possibly misguided) desire to
continue producing stats output into the viewer log in the same order it
always used to be produced. There is no evidence that anyone cares about the
order of those stats in the log; there is no other use case for preserving
order. At Monty's recommendation, eliminate generating and storing the
array-of-pair-arrays form: directly store LLSD::Map.