value_frequency.js - slightly change data representations in `compute_stats_for_string_keys_from_value_frequencies` function
value_frequency.js - slightly change data representations in `compute_stats_for_string_keys_from_value_frequencies` function

This is to prepare for an upcoming patch which will compute the amount of space
we can potentially save by adopting a smarter way of storing the same value
strings.

Signed-off-by: Pang Yan Han <22594c345857f664c61cd411f607c4d40a9d4af9@gmail.com>

yanhan/cljs-nusmods,yanhan/cljs-nusmods,yanhan/cljs-nusmods