fichier: 1fichier support - fixes #2908

This was started by Fionera, finished off by Laura with fixes and more
docs from Nick.

Co-authored-by: Fionera <fionera@fionera.de>
Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>

backend/fichier/replace_test.go

@@ -0,0 +1,24 @@
+package fichier
+
+import "testing"
+
+func TestReplace(t *testing.T) {
+	for _, test := range []struct {
+		in  string
+		out string
+	}{
+		{"", ""},
+		{"abc 123", "abc 123"},
+		{"\"'<>/\\$`", `＂＇＜＞/＼＄｀`},
+		{" leading space", "␠leading space"},
+	} {
+		got := replaceReservedChars(test.in)
+		if got != test.out {
+			t.Errorf("replaceReservedChars(%q) want %q got %q", test.in, test.out, got)
+		}
+		got2 := restoreReservedChars(got)
+		if got2 != test.in {
+			t.Errorf("restoreReservedChars(%q) want %q got %q", got, test.in, got2)
+		}
+	}
+}