Utilize Vector{TaskArg} for serialize_args

[omus]

In #72 we ended up appending the TaskArgByValue and TaskArgByReference instances to a Vector{Any} even when TaskArgByValue <: TaskArg and TaskArgByReference <: TaskArg. This is due to how CxxWrap handles convert:

julia> task_arg
ray_core_worker_julia_jll.TaskArgByValueAllocated(Ptr{Nothing} @0x0000600000e24be0)

julia> x = ray_jll.TaskArg[]
ray_core_worker_julia_jll.TaskArg[]

julia> push!(x, task_arg)
ERROR: MethodError: Cannot `convert` an object of type CxxRef{ray_core_worker_julia_jll.TaskArg} to an object of type ray_core_worker_julia_jll.TaskArg

Closest candidates are:
  convert(::Type{ray_core_worker_julia_jll.TaskArg}, ::ray_core_worker_julia_jll.TaskArgDereferenced)
   @ ray_core_worker_julia_jll ~/.julia/packages/CxxWrap/aXNBY/src/CxxWrap.jl:687
  convert(::Type{ray_core_worker_julia_jll.TaskArg}, ::ray_core_worker_julia_jll.TaskArgAllocated)
   @ ray_core_worker_julia_jll ~/.julia/packages/CxxWrap/aXNBY/src/CxxWrap.jl:686
  convert(::Type{ray_core_worker_julia_jll.TaskArg}, ::T) where T<:ray_core_worker_julia_jll.TaskArg
   @ ray_core_worker_julia_jll ~/.julia/packages/CxxWrap/aXNBY/src/CxxWrap.jl:682
  ...

Stacktrace:
 [1] setindex!(A::Vector{ray_core_worker_julia_jll.TaskArg}, x::CxxRef{ray_core_worker_julia_jll.TaskArg}, i1::Int64)
   @ Base ./array.jl:969
 [2] push!(a::Vector{ray_core_worker_julia_jll.TaskArg}, item::ray_core_worker_julia_jll.TaskArgByValueAllocated)
   @ Base ./array.jl:1062
 [3] top-level scope
   @ REPL[15]:1

I believe this is due to how C++ handles class inheritance. If you have a std::vector<TaskArg> then all instances of that class are just TaskArg. We can see this in Julia by using a StdVector:

julia> task_arg
ray_core_worker_julia_jll.TaskArgByValueAllocated(Ptr{Nothing} @0x0000600000e24be0)

julia> task_arg_ptrs = StdVector{CxxPtr{ray_jll.TaskArg}}()
0-element CxxWrap.StdLib.StdVectorAllocated{CxxPtr{ray_core_worker_julia_jll.TaskArg}}

julia> push!(task_arg_ptrs, CxxRef(task_arg))
1-element CxxWrap.StdLib.StdVectorAllocated{CxxPtr{ray_core_worker_julia_jll.TaskArg}}:
 CxxPtr{ray_core_worker_julia_jll.TaskArg}(Ptr{ray_core_worker_julia_jll.TaskArg} @0x0000600000e24be0)

I think what should probably happen is that when using a Julia Vector the types should behave more like how Julia typicaly does where a Vector{TaskArg} contains the concrete subtypes of TaskArgByValue and TaskArgByReference and when using a StdVector the types are automatically casted. Doing this would require CxxWrap to have it's own say cconvert function which works differently from the default convert (note: the default push! calls convert).