Fix codepoint byte counting in slice, closes #11461

Author: josevalim

lib/elixir/lib/string.ex

@@ -2736,7 +2736,7 @@ defmodule String do
     end
   end
 
-  defp codepoint_byte_size(cp) when cp <= 0x00FF, do: 1
+  defp codepoint_byte_size(cp) when cp <= 0x007F, do: 1
   defp codepoint_byte_size(cp) when cp <= 0x07FF, do: 2
   defp codepoint_byte_size(cp) when cp <= 0xFFFF, do: 3
   defp codepoint_byte_size(_), do: 4

lib/elixir/test/elixir/string_test.exs

@@ -652,6 +652,7 @@ defmodule StringTest do
     assert String.slice("あいうえお", -10..-15) == ""
     assert String.slice("hello あいうえお Unicode", 8..-1) == "うえお Unicode"
     assert String.slice("abc", -1..14) == "c"
+    assert String.slice("a·̀ͯ‿.⁀:", 0..-2) == "a·̀ͯ‿.⁀"
 
     assert_raise FunctionClauseError, fn ->
       String.slice(nil, 0..1)