Improve juliacall startup time?

Currently, it takes more than 2 seconds to load the package `juliacall` in Python, before importing any Julia package or running Julia code:
```
$ time python -c "from juliacall import Main as jl"

real 0m2.355s
user 0m3.104s
sys 0m0.156s
```

This can be a disadvantage compared with calling C++ instead of Julia to offload heavy computations outside Python, depending on the workflow. Is there a way to compile a Julia system image to make the startup much faster?

Start by reproducing the reported command, `python -c "from juliacall import Main as jl"`, and inspect the juliacall import and initialization path. Determine which startup work dominates the more-than-two-second delay and whether a Julia system image can address it. Done means a documented, measured startup improvement without breaking Python-to-Julia initialization.

索引モデルが issue の本文から書いたものです。