feat(sterni/nix/utf8): implement UTF-8 encoding

This implementation is still a bit rough as it doesn't check if the
produced string is valid UTF-8 which may happen if an invalid Unicode
codepoint is passed.

Change-Id: Ibaa91dafa8937142ef704a175efe967b62e3ee7b

users/sterni/nix/utf8/tests/default.nix

@@ -113,9 +113,19 @@ let
         randomUnicode
       ]));
 
+  testDecodingEncoding = it "checks that decoding and then encoding forms an identity"
+    (builtins.map
+      (s: assertEq "Decoding and then encoding “${s}” yields itself"
+        (utf8.encode (utf8.decode s)) s)
+      (lib.flatten [
+        glassSentences
+        randomUnicode
+      ]));
+
 in
   runTestsuite "nix.utf8" [
     testFailures
     testAscii
     testDecoding
+    testDecodingEncoding
   ]