Support ProductNamedTupleDistribution

[penelopeysm]

julia> using Distributions, Bijectors

julia> dist = product_distribution((a = Normal(), b = Normal()))
ProductNamedTupleDistribution{(:a, :b)}(
a: Normal{Float64}(μ=0.0, σ=1.0)
b: Normal{Float64}(μ=0.0, σ=1.0)
)

julia> bijector(dist)
ERROR: MethodError: no method matching bijector(::Distributions.ProductNamedTupleDistribution{(:a, :b), Tuple{Normal{…}, Normal{…}}, Continuous, Float64})
The function `bijector` exists, but no method is defined for this combination of argument types.

I think we need to add a case here:

Bijectors.jl/src/transformed_distribution.jl

Lines 75 to 90 in fbaf783

	# Container distributions.
	bijector(d::DiscreteUnivariateDistribution) = identity
	bijector(d::DiscreteMultivariateDistribution) = identity
	bijector(d::ContinuousUnivariateDistribution) = TruncatedBijector(minimum(d), maximum(d))
	bijector(d::Product{Discrete}) = identity
	function bijector(d::Product{Continuous})
	D = eltype(d.v)
	return if has_constant_bijector(D)
	elementwise(bijector(d.v[1]))
	else
	# FIXME: This is not great. Should use something like
	# `Stacked(map(bijector, d.v))` instead.
	# TODO: Specialize. F.ex. for FillArrays.jl we can do much better.
	TruncatedBijector(_minmax(d.v)...)
	end
	end

And I think that StackedBijector already gives us most of the actual code that we need, although we might need additional code to marshal the result from/to a NamedTuple(?).