Overhaul unicode parsing

It now instead iterates the actual unicode code points. This is better
than what it was previously doing but it is still not entirely correct
w.r.t to unicode sequences.

This handling of unicode code points does however make it slightly
easier to handle UTF-16 if needed in the future.

This also adds some long needed tests for buffer methods.

dged.nix

@@ -10,6 +10,8 @@
 , valgrind
 , linkFarm
 , fetchFromGitHub
+, glibcLocalesUtf8
+, strace
 }:
 stdenv.mkDerivation {
   name = "dged";
@@ -32,6 +34,10 @@ stdenv.mkDerivation {
     bmake docs
   '';
 
+  # needed for tests to work in sandboxed builds
+  LANG = "en_US.UTF-8";
+  LOCALE_ARCHIVE = "${glibcLocalesUtf8}/lib/locale/locale-archive";
+
   TREESITTER_GRAMMARS = with tree-sitter-grammars;
     linkFarm "tree-sitter-grammars" rec {
       "bash" = tree-sitter-bash;