Add tests for type stability

[mhauru]

This came up in the context of Turing.jl's Enzyme PR and the Gibbs performance regression: Would be nice to have extensive tests that give us confidence that all the core parts of DPPL are type stable.

[mhauru]

@AoifeHughes, this one could be a good one, maybe not right now but soon, if you want to learn to understand type stability in Julia. (Is there a good source that explains what type stability is? Docs are pretty terse.) The implementation here is very simple: Put some calls for Test.@inferred in the tests for some core functions. This might even be as simple as adding Test.@inferred in a bunch of places in existing tests and doing nothing else. However, knowing which functions to use it for requires knowing a bit how DynamicPPL works, so if you want to take this on let's schedule a chat about that. Also, if some of those tests turn out to not pass, then we could have some exciting type stability bugs to fix.

[AoifeHughes]

Okie dokie, thanks :)

[yebai]

Either JET.jl or DispatchDoctor.jl could be good solutions.