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/internal/language/compact.go

@@ -0,0 +1,29 @@
+// Copyright 2018 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 language
+
+// CompactCoreInfo is a compact integer with the three core tags encoded.
+type CompactCoreInfo uint32
+
+// GetCompactCore generates a uint32 value that is guaranteed to be unique for
+// different language, region, and script values.
+func GetCompactCore(t Tag) (cci CompactCoreInfo, ok bool) {
+	if t.LangID > langNoIndexOffset {
+		return 0, false
+	}
+	cci |= CompactCoreInfo(t.LangID) << (8 + 12)
+	cci |= CompactCoreInfo(t.ScriptID) << 12
+	cci |= CompactCoreInfo(t.RegionID)
+	return cci, true
+}
+
+// Tag generates a tag from c.
+func (c CompactCoreInfo) Tag() Tag {
+	return Tag{
+		LangID:   Language(c >> 20),
+		RegionID: Region(c & 0x3ff),
+		ScriptID: Script(c>>12) & 0xff,
+	}
+}