Speeding up Soss loading

[sethaxen]

As mentioned in #105, I'd like to get Soss's time-to-first-model down. Besides making it easier to develop, this will also make Soss more attractive as a dependency for other projects.

On my machine, removing Plots.jl (in #115) takes Soss from ~26s load time to ~19s load time. In a brief test, it looks like making SymPy.jl an optional dependency (via Requires.jl) would reduce load time further to ~14s! That's a pretty awesome speed-up for removing 2 dependencies.

For comparison, Turing loads in ~33s, and Gen loads in ~6s.

Are there other heavy dependencies that provide features that won't be needed by every user and could potentially be made optional?