Reduce minetest.after time complexity and provide ordering guarantee

---------

Co-authored-by: Lars Mueller <appgurulars@gmx.de>

builtin/common/tests/after_spec.lua

@@ -0,0 +1,113 @@
+_G.core = {}
+_G.vector = {metatable = {}}
+
+dofile("builtin/common/vector.lua")
+dofile("builtin/common/misc_helpers.lua")
+
+function core.get_last_run_mod() return "*test*" end
+function core.set_last_run_mod() end
+
+local do_step
+function core.register_globalstep(func)
+	do_step = func
+end
+
+dofile("builtin/common/after.lua")
+
+describe("after", function()
+	it("executes callbacks when expected", function()
+		local result = ""
+		core.after(0, function()
+			result = result .. "a"
+		end)
+		core.after(1, function()
+			result = result .. "b"
+		end)
+		core.after(1, function()
+			result = result .. "c"
+		end)
+		core.after(2, function()
+			result = result .. "d"
+		end)
+		local cancel = core.after(2, function()
+			result = result .. "e"
+		end)
+		do_step(0)
+		assert.same("a", result)
+
+		do_step(1)
+		assert.same("abc", result)
+
+		core.after(2, function()
+			result = result .. "f"
+		end)
+		core.after(1, function()
+			result = result .. "g"
+		end)
+		core.after(-1, function()
+			result = result .. "h"
+		end)
+		cancel:cancel()
+		do_step(1)
+		assert.same("abchdg", result)
+
+		do_step(1)
+		assert.same("abchdgf", result)
+	end)
+
+	it("defers jobs with delay 0", function()
+		local result = ""
+		core.after(0, function()
+			core.after(0, function()
+				result = result .. "b"
+			end)
+			result = result .. "a"
+		end)
+		do_step(1)
+		assert.same("a", result)
+		do_step(1)
+		assert.same("ab", result)
+	end)
+
+	it("passes arguments", function()
+		core.after(0, function(...)
+			assert.same(0, select("#", ...))
+		end)
+		core.after(0, function(...)
+			assert.same(4, select("#", ...))
+			assert.same(1, (select(1, ...)))
+			assert.same(nil, (select(2, ...)))
+			assert.same("a", (select(3, ...)))
+			assert.same(nil, (select(4, ...)))
+		end, 1, nil, "a", nil)
+		do_step(0)
+	end)
+
+	it("rejects invalid arguments", function()
+		assert.has.errors(function() core.after() end)
+		assert.has.errors(function() core.after(nil, nil) end)
+		assert.has.errors(function() core.after(0) end)
+		assert.has.errors(function() core.after(0, nil) end)
+		assert.has.errors(function() core.after(nil, function() end) end)
+		assert.has.errors(function() core.after(0 / 0, function() end) end)
+	end)
+
+	-- Make sure that the underlying heap is working correctly
+	it("can be abused as a heapsort", function()
+		local t = {}
+		for i = 1, 1000 do
+			t[i] = math.random(100)
+		end
+		local sorted = table.copy(t)
+		table.sort(sorted)
+		local i = 0
+		for _, v in ipairs(t) do
+			core.after(v, function()
+				i = i + 1
+				assert.equal(v, sorted[i])
+			end)
+		end
+		do_step(math.max(unpack(t)))
+		assert.equal(#t, i)
+	end)
+end)