Dynamic symbols on Unix

[filmor]

Implements #648.

We could go even further and drop rustler-sys entirely.

[filmor]

While this works, there seems to be a non-negligible performance impact. I'll try to find out where it comes from.

[filmor]

I looked more into the performance impact, and I think it's compile-time only, at least on Linux. I also compared the assemblies and from what I can see, the generated code for the function calls looks very similar (but still too different to just claim it will execute identically). My current working assumption is that the differences in CI runtimes can be attributed to the CI hosts and potentially a slightly slower startup (as repeated dlsym might be slower than linking at load time).

Please review, I will in parallel try to get a pure macro based variant working in another PR.