Document and move matchingvalue

[mhauru]

compiler.jl has the function matchingvalue, which does conversion of the value given for the purposes of using it in model arguments. It's non-trivial to read and needs documenting, perhaps a more descriptive name, and maybe should be moved to model.jl.

Need for this originally noted here: https://github.com/TuringLang/DynamicPPL.jl/pull/793/files#r1936343226

[penelopeysm]

eltype's docstring suggests that it could be cleaned up too:

DynamicPPL.jl/src/abstract_varinfo.jl

Lines 343 to 364 in 29a6c7e

	"""
	eltype(vi::AbstractVarInfo, spl::Union{AbstractSampler,SampleFromPrior}
	Determine the default `eltype` of the values returned by `vi[spl]`.
	!!! warning
	This should generally not be called explicitly, as it's only used in
	[`matchingvalue`](@ref) to determine the default type to use in place of
	type-parameters passed to the model.
	This method is considered legacy, and is likely to be deprecated in the future.
	"""
	function Base.eltype(vi::AbstractVarInfo, spl::Union{AbstractSampler,SampleFromPrior})
	T = Base.promote_op(getindex, typeof(vi), typeof(spl))
	if T === Union{}
	# In this case `getindex(vi, spl)` errors
	# Let us throw a more descriptive error message
	# Ref https://github.com/TuringLang/Turing.jl/issues/2151
	return eltype(vi[spl])
	end
	return eltype(T)
	end

I'm not immediately proposing that I'll do this, but it would be interesting to just delete this method and see what breaks. (Locally, of course.)