Add utf8 (#1817)

* add utf8.h

* add utf8

packages/b/branchless-utf8/xmake.lua

@@ -0,0 +1,15 @@
+package("branchless-utf8")
+    set_kind("library", {headeronly = true})
+    set_homepage("https://github.com/skeeto/branchless-utf8")
+    set_description("Branchless UTF-8 decoder")
+
+    add_urls("https://github.com/skeeto/branchless-utf8.git")
+    add_versions("2022.08.30", "e4d82fd5ddae98658f8c129006e3bc5acd0ae9f1")
+
+    on_install(function (package)
+        os.cp("*.h", package:installdir("include"))
+    end)
+
+    on_test(function (package)
+        assert(package:has_cfuncs("utf8_decode", {includes = "utf8.h"}))
+    end)