bug#65491: [PATCH] Improve performance allocating vectors

[Mattias Engdegård]

> - Reducing the `vector_free_lists` array to its actually used first half does 
> improve performance a bit, even with subsequent scanning speed-ups.

> - The previously mentioned hack where scanning begins in the most recently 
> added bucket is surprisingly effective, often even more so than a bitmap, but 
> I'm wary of it being brittle. Need more measurements.

These two changes were simple, fairly effective, and found to be reasonably 
safe (in the sense that they are unlikely to make performance worse), so I 
pushed them to master. This way we should be better off even if no further 
progress is made.

As mentioned, a bitmap is essentially as good as or better than the 
last-used-index heuristic but I went with the latter because it involved a lot 
less code. This can be changed.

But it is possible to do better: size-homogeneous blocks, where each block only 
houses vectors of a single size, have the major benefit that 
live_small_vector_holding no longer needs to traverse half the block on average 
but becomes is done in constant time. Sweeping, always performance-critical 
since it traverses both living and dead objects, becomes slightly faster. There 
is also no free-list array scanning at all.

Benchmarks suggest that larger homogeneous blocks (32 KiB, say) would be 
beneficial, but we need to look at the potential extra fragmentation caused by 
allocating a big block for each odd vector size.