Avoid most dynamic allocation

[ALSchwalm]

In order to ensure more deterministic performance, we must avoid dynamic allocation pretty much anywhere after launch the VCPUs. There are currently a lot of places that allocate strings and vecs and such. We should move away from these to ArrayVec and friends.

[francis-starlab]

I can't assign myself. So this comment consists of me assigning myself.

[ALSchwalm]

Future work on this should be based on #114

Other outstanding things are, for example, all String allocations (particularly in errors)