Implement `to_varname_dict` better

[penelopeysm]

This is a very bad implementation that I did a while ago:

DynamicPPL.jl/src/utils.jl

Lines 851 to 854 in 08fffa2

	# Convert (x=1,) to Dict(@varname(x) => 1)
	function to_varname_dict(nt::NamedTuple)
	return Dict{VarName,Any}(VarName{k}() => v for (k, v) in pairs(nt))
	end

It should really make sure that at least the value type, and possibly the key type, is as concrete as possible.