Normalize local usernames using PRECIS

This patch implements local username normalization using PRECIS
(https://tools.ietf.org/html/rfc7564,
https://tools.ietf.org/html/rfc7613)

It makes chasquid accept local email and authentication regardless of
the case. It covers both userdb and aliases.

Note that non-local usernames remain untouched.

internal/normalize/normalize.go

@@ -0,0 +1,31 @@
+// Package normalize contains functions to normalize usernames and addresses.
+package normalize
+
+import (
+	"blitiri.com.ar/go/chasquid/internal/envelope"
+	"golang.org/x/text/secure/precis"
+)
+
+// User normalices an username using PRECIS.
+// On error, it will also return the original username to simplify callers.
+func User(user string) (string, error) {
+	norm, err := precis.UsernameCaseMapped.String(user)
+	if err != nil {
+		return user, err
+	}
+
+	return norm, nil
+}
+
+// Name normalices an email address using PRECIS.
+// On error, it will also return the original address to simplify callers.
+func Addr(addr string) (string, error) {
+	user, domain := envelope.Split(addr)
+
+	user, err := User(user)
+	if err != nil {
+		return addr, err
+	}
+
+	return user + "@" + domain, nil
+}