Ben Gamari authored Jun 22, 2020 and Marge Bot committed Jun 25, 2020

While looking at #18348 I noticed that the treatment of HashLists are a
bit more complex than necessary (which lead to some initial confusion on
my part). Specifically, we allocate HashLists in chunks. Each chunk
allocation makes two allocations: one for the chunk itself and one for a
HashListChunk to link together the chunks for the purposes of freeing.

Simplify this (and hopefully make the relationship between these
clearer) but allocating the HashLists and HashListChunk in a single
malloc. This will both make the implementation easier to follow and
reduce C heap fragmentation.

Note that even after this patch we fail to bound the size of the free
HashList pool. However, this is a separate bug.