perf: spatial hashing for InterestManagementBase

[imerr]

This is very much a work in progress and NEEDS changing before it is ready to merge (no runtime visibility changes and probably needs code cleaning up/commenting)

In my past testing, it's 4x faster for me In @ninjakickja's recent testing ("simulation") it's 2.5x faster than the current one

Depends on/see #3379

[imerr]

Oh, the interest management test setup does not play well with this (from memory):

• aoi.OnSpawned is not called since test identities are spawned before the aoi is assigned/created
• the manual spawn assignments in the base NT add each identity twice

So that would need improving too

[imerr]

Ideally we'd just do a 1:1 non-breaking replacement for the normal spatial hashing here so users don't have to care, that should be doable?