Enable keyword arguments for particle methods

[penelopeysm]

Note

This PR requires some changes to AdvancedPS. TuringLang/AdvancedPS.jl#118

It also needs the following Libtask patch: TuringLang/Libtask.jl#198

Until both are merged CI is likely to fail.

This PR allows models with keyword arguments to be run with SMC / PG.

Example:

julia> using Turing

julia> @model function m(y; n=0)
           x ~ Normal(n)
           y ~ Normal(x)
       end
m (generic function with 2 methods)

julia> mean(sample(m(5.0), PG(20), 1000))
[...]
ERROR: Models with keyword arguments need special treatment to be used with particle methods. Please run:

    using Libtask; Libtask.@might_produce(m)

before sampling from this model with particle methods.

Stacktrace:
[...]

julia> using Libtask; Libtask.@might_produce(m)

julia> mean(sample(m(5.0), PG(20), 1000))
[...]
Mean
  parameters      mean
      Symbol   Float64

           x    2.4155

Closes #2007.

[penelopeysm]

I would be open to re-exporting this, or something similar, from Turing. However I think it's not hugely important.