Is there a function that returns an array of all equipped (currently in use) and unequipped (in the hero's inventory) artifacts?

No, but it's easy to create.

function GetAllArtefacts(hero)
	local array = {};
	for artefact = 1, 96 do
		if HasArtefact(hero, artefact, 1) then
			array[artefact] = 2;
		elseif HasArtefact(hero, artefact, 0) then
			array[artefact] = 1;
		else
			array[artefact] = 0;
		end;
	end;
	return array;
end;

For example, this function will return an array where the index = the artifact number, and the value = 0, 1, 2, respectively: none, present, equipped.

function GetAllArtefacts(hero)
	local array = {[0] = {}, [1] = {}};
	for artefact = 1, 96 do
		if HasArtefact(hero, artefact, 1) then
			array[1][length(array[1]) + 1] = artefact;
		elseif HasArtefact(hero, artefact, 0) then
			array[0][length(array[0]) + 1] = artefact;
		end;
	end;
	return array;
end;

And this one will return an array where array[0] is an array of unequipped artifacts, and array[1] is an array of equipped artifacts. However, in both cases, if there are ≥2 artifacts of the same type, both functions will say that they are equipped.