Implement `prefix` for Distributions.jl and depreciate `NamedDist`.

[yebai]

Let's use prefix to replace the slightly redundant NamedDist:

DynamicPPL.jl/src/distribution_wrappers.jl

Lines 5 to 12 in d0cfaaf

	"""
	A named distribution that carries the name of the random variable with it.
	"""
	struct NamedDist{variate,support,Td<:Distribution{variate,support},Tv<:VarName} <:
	Distribution{variate,support}
	dist::Td
	name::Tv
	end

x ~ prefix(Normal(), "var_`") # store `var_x` instead of `x` in `varinfo`. 

[penelopeysm]

Why do we even need NamedDist (or any mechanism to control the name)? Functionally, is there any difference between

var_x ~ Normal()
x = var_x

and

x ~ prefix(Normal(), "var_")

? I guess the former introduces an extra variable into the local scope, but I can't imagine why that should be a problem.

[penelopeysm]

Ah, I see, it lets people programmatically generating variable names. Urk.

I think it would make far more sense if this could be implemented on the LHS of the tilde, rather than the RHS. But I see that this has been discussed before.