ProbProg: Making trace an operand

[sbrantq]

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[sbrantq]

@wsmoses So I added GC.@preservings to fix the deadlock issue with the ConcreteRArray that specifies rng.seed (for context GC triggered by EnzymeJaXMapSymbol'd functions somehow tries to free only(rng.seed.data).buffer):

Reactant.jl/src/probprog/Modeling.jl

Lines 283 to 285 in 9bd1dee

	seed_buffer = only(rng.seed.data).buffer
	GC.@preserve seed_buffer constraint begin
	trace, _, _ = compiled_fn(rng, constraint_ptr, args...)

Also GC triggered by mapped functions in src/probprog/FFI.jl can cause jl_gc_wait_for_the_world to wait forever for the main Julia thread which is waiting during Base.convert to Julia Array, like this one

Reactant.jl/src/probprog/Modeling.jl

Line 292 in 9bd1dee

trace = unsafe_pointer_to_objref(Ptr{Any}(Array(trace)[1]))

So I added

Reactant.jl/src/probprog/Modeling.jl

Lines 287 to 289 in 9bd1dee

	while !isready(trace)
	yield()
	end

(Adding trivial Base.isready for ConcretePJRTArray and ConcreteIFRTArray).

Does that sound like an ok-ish fix, or am i missing something?