Implement basic IDNA support

This patch implements the first steps of support for IDNA (Internationalized
Domain Names for Applications).

Internally, we maintain the byte-agnostic representation, including
configuration.

We support receiving IDNA mail over SMTP, which we convert to unicode for
internal handling.

Local deliveries are still kept agnostic.

For sending over SMTP, we use IDNA for DNS resolution, but there are some
corner cases pending in the SMTP courier that are tied with SMTPUTF8 and will
be fixed in subsequent patches.

internal/envelope/envelope.go

@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"strings"
 
+	"golang.org/x/net/idna"
+
 	"blitiri.com.ar/go/chasquid/internal/set"
 )
 
@@ -48,3 +50,27 @@ func AddHeader(data []byte, k, v string) []byte {
 	header := []byte(fmt.Sprintf("%s: %s\n", k, v))
 	return append(header, data...)
 }
+
+// Take an address with a potentially unicode domain, and convert it to ASCII
+// as per IDNA.
+// The user part is unchanged.
+func IDNAToASCII(addr string) (string, error) {
+	if addr == "<>" {
+		return addr, nil
+	}
+	user, domain := Split(addr)
+	domain, err := idna.ToASCII(domain)
+	return user + "@" + domain, err
+}
+
+// Take an address with an ASCII domain, and convert it to Unicode as per
+// IDNA.
+// The user part is unchanged.
+func IDNAToUnicode(addr string) (string, error) {
+	if addr == "<>" {
+		return addr, nil
+	}
+	user, domain := Split(addr)
+	domain, err := idna.ToUnicode(domain)
+	return user + "@" + domain, err
+}