Randomise the offset where we start allocating objects in a run of spans

[pkhuong]

Given guard pages between runs of spans, we'll want to sometimes make objects start right after a guard page, or end flush before a guard page. We can implement that by randomly choosing a different "colour" for each run.

We'll also allocate objects sequentially, so we'll want to randomise the initial index for bump allocation. Should we have two indices, or add an initial index offset modulo object capacity per run?