normalize localparts with unicode nfc when parsing

both when parsing our configs, and for incoming on smtp or in messages.
so we properly compare things like é and e+accent as equal, and accept the
different encodings of that same address.

testdata/smtp/domains.conf

@@ -8,6 +8,11 @@ Accounts:
 			mjl@mox.example: nil
 			mjl@mox2.example: nil
 			""@mox.example: nil
+			# ascii o, we'll check that greek & cyrillic lookalike isn't accepted
+			o@mox.example: nil
+			# ohm sign, \u2126
+			Ω@mox.example: nil
+			tést@mox.example: nil
 		JunkFilter:
 			Threshold: 0.9
 			Params: