Fix get_in errors

[willcohen]

When trying to compile on macOS, hitting #126. This PR replaces uses of get_in with the wrapped direct access.

[edderiofer]

This appears to compile fine on my end (i.e. on Windows, after making the necessary undocumented changes for compiling on Windows). Now we just need someone on Linux to attempt to compile it.

[willcohen]

Docker and nix environments added here for reproducibility, and cross-platform compatibility for windows/mac/linux.

[EpicOrange]

I looked this over and it looks good! But I'm going to leave this PR open for now, and can direct ppl to it if they have compilation issues. Mostly it's because I don't want to do the work of maintaining a nix lockfile + dockerfiles quite yet when the structure of the repo is still kind of questionable, e.g.

• There's a Python dependency that will be removed (I've already written a replacement)
• There's a node/npm dependency that I want to remove but haven't figured out
• Many directories and files are completely unused (see directory structure in README), it's Phoenix framework starter code that I haven't ripped out
• Unfortunately there's not really a dev and prod environment. There is just a dev environment, and a prod environment I've never touched
• There are a few Elixir libraries (z3 bindings, jq bindings) that are """""temporarily""""" copy-pasted wholesale into this repository (haha...)
• Nothing has been migrated to newer versions of Elixir yet (the Elixir discord doesn't seem to have any ideas for this either actually so idk if this matters)

I'd like to address at least some of these before doing any Dockerfile stuff so we don't have to keep making new Dockerfiles :)

[EpicOrange]

I've installed the latest Elixir 1.19.5 and it seems like get_in works perfectly fine. What version(s) of Elixir are we running where get_in errors out?