Directly use libjulia

I'm not suggesting we do this anytime soon, but eventually I think the `julialauncher` binary here could take the place of https://github.com/JuliaLang/julia/tree/master/cli? That way we would speed up startup times.

CC @staticfloat

Start by reading the julialauncher implementation in this repository and comparing it with the Julia CLI at the linked JuliaLang/julia path. Determine whether replacing that CLI is feasible and what startup-time improvement would count as success; the issue does not define a concrete scope or completion test.

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