util.format: Ensure sanitation of strings passed to wrong format

Ie. log("debug", "%d", "\1\2\3") should not result in garbage.

Also optimizing for the common case of ASCII string passed to %s and
early returns everywhere.

Returning nil from a gsub callback keeps the original substring.

spec/util_format_spec.lua

@@ -18,6 +18,7 @@ describe("util.format", function()
 
 		it("escapes ascii control stuff", function ()
 			assert.equal("␁", format("%s", "\1"));
+			assert.equal("[␁]", format("%d", "\1"));
 		end);
 
 		it("escapes invalid UTF-8", function ()