Replace httpstan wrapper with BridgeStan

[simeoncarstens]

Stan support in Chainsail currently relies on wrapper around httpstan. So for every log-probability / gradient evaluation, the wrapper sends a HTTP request to the httpstan API that runs in a separate Docker container. This is very, very slow and very, very clunky.

Recently, BrigeStan was released. It allows

efficient in-memory access through Python, Julia, and R to the methods of a Stan model, including log densities, gradients, Hessians, and constraining and unconstraining transforms

and is exactly what we need. I did some preliminary tests and using BridgeStan yields a x1000 speed-up over using the existing httpstan wrapper 🤯 plus it might be easier to maintain; we could just bake it in as a system dependency into the user code container. My experiments stopped there because of the trouble of developing this under NixOS - BridgeStan requires stanc (the Stan compiler) and downloads binaries for that, which have an hardcoded RPATH, and furthermore, it requires GCC + dependencies. So under NixOS, without having packaged BridgeStan properly, developing this feature is very tiresome.

So the goal would be to replace the httpstan wrapper linked above with a wrapper around BridgeStan. Note, though, that the Stan model still has to be compiled before the log-probability and its gradient can be evaluated via BridgeStan. That would likely need to be done in the constructor of the wrapper.