Vectorize borrow/supply computation

[danhper]

compute_user_position is currently fine for a single user but it is very slow for many users.
It would be good to have a vectorized version that can compute n users at a time.

[richardgreg]

Hi @danhper, I'd like to have a go at this

[richardgreg]

@danhper are you looking for a solution such as instead of a single user, compute_user_position takes in a list of users and returns, say a hashmap of user positions keyed to the user id?