Use a serialization library that is bidirectional between julia versions

It would be nice to move away from the standard library Serialization, as it only guarantees that newer julia versions can read files written by older ones, and not the other way. Bi-directionality is needed.

Based on the recommendations in https://fluxml.ai/Flux.jl/stable/saving/, options are https://github.com/JuliaIO/JLD2.jl and https://github.com/JuliaIO/BSON.jl. Perhaps lightweightness should be a priority. On my system, in a temp environment, I get
```
julia> @time using JLD2
0.312673 seconds (316.65 k allocations: 18.135 MiB)

julia> @time using BSON
0.002721 seconds (3.12 k allocations: 288.133 KiB)
```
Also, BSON.jl has no dependencies, so I guess there is less that can go wrong there? It seems appropriate to use a minimal low-dependency library for serialization in the context of RemoteREPL, so I suggest BSON.jl. Is this a direction that can be agreed on?