Pre-allocate vectors

[Kixiron]

Pre-allocates more vectors in the hopes of reducing startup times and memory usage

[frankmcsherry]

Thanks! Generally looks good, in particular the ref and ref mut removal, and the with_capacity calls which much more clearly indicate intent. There is some cargo fmt line noise that it would be great to ditch (the code is formatted for a wide and short display, and cargo fmt's opinion that I shouldn't have that display is ...).

[Kixiron]

I've been looking into how to speed up dataflow construction (kinda in the vein of #308) and one of the things that I've noticed is that there's just a lot of allocations, but I'm not entirely sure how to go about removing/reducing most of them. Preallocation is a nice low hanging fruit, so I figured I'd try it out and see what happens

[frankmcsherry]

I'm up for looking in to what is the source of the slowness. I've profiled it for some microbenchmarks, and .. it was smeared out a bit. There are several places where allocation could be more lazy (e.g. data buffers between workers) and where fewer allocations could be used (e.g. in reachability). But, if you end up with some traces that incriminate things, I'm up for talking through what is needed and what is not.

[Kixiron]

Sure thing, I'll look into it these next few weeks and see if I can't uncover anything!