prevent unicode-confusion in password by applying PRECIS, and username/email address by applying unicode NFC normalization

an é (e with accent) can also be written as e+\u0301. the first form is NFC,
the second NFD. when logging in, we transform usernames (email addresses) to
NFC. so both forms will be accepted. if a client is using NFD, they can log
in too.

for passwords, we apply the PRECIS "opaquestring", which (despite the name)
transforms the value too: unicode spaces are replaced with ascii spaces. the
string is also normalized to NFC. PRECIS may reject confusing passwords when
you set a password.

vendor/golang.org/x/text/secure/precis/doc.go

@@ -0,0 +1,14 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package precis contains types and functions for the preparation,
+// enforcement, and comparison of internationalized strings ("PRECIS") as
+// defined in RFC 8264. It also contains several pre-defined profiles for
+// passwords, nicknames, and usernames as defined in RFC 8265 and RFC 8266.
+//
+// BE ADVISED: This package is under construction and the API may change in
+// backwards incompatible ways and without notice.
+package precis // import "golang.org/x/text/secure/precis"
+
+//go:generate go run gen.go gen_trieval.go