Use AllocCheck.jl in tests

[mhauru]

@willtebbutt and @sunxd3 brought up https://github.com/JuliaLang/AllocCheck.jl, a tool that checks whether a function may cause memory allocations. We could use it in tests kinda like how we use @inferred, to check that things that aren't expected allocate memory, don't.

[yebai]

I think making Turing.jl allocation free is very challenging.

[willtebbutt]

I'm not sure that @mhauru is suggesting we could make the entire thing allocation-free, just specific pieces that we're confident ought not to allocate. I agree that making the whole thing allocation-free is probably not currently on the table.