lib/fs: Add case insensitivity to MtimeFS

This is step one of a hundred fifty on the path to case insensitivity.
It brings in the basic case folding mechanism and adds it to the
mtimefs, as this is something outside the fileset that touches stuff in
the database based on name. No effort to convert or handle existing
entries when the insensitivity is changed, I don't think we need it...

Useless by itself but includes tests and will reduce the review load
along the way.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4521

lib/fs/folding.go

@@ -0,0 +1,19 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package fs
+
+import (
+	"unicode"
+)
+
+func UnicodeLowercase(s string) string {
+	rs := []rune(s)
+	for i, r := range rs {
+		rs[i] = unicode.ToLower(unicode.ToUpper(r))
+	}
+	return string(rs)
+}