Check arguments before looking for the mnemonic (closes #79)

The key index, --words, the --to recipients and the input file of
age encrypt and age decrypt were checked only after the mnemonic had
been obtained, so a run that could not succeed still prompted for it or
ran the mnemonic command. options.Index now refuses an index above
derive.MaxIndex, which covers every command group; the mnemonic command
calls the new childmnemonic.CheckWords first; age encrypt reads its
recipients with the new agekey.Recipients, and both age commands open
their input, before the key is derived. The README says which checks
come first.

Judgement call: the -o path is still looked at after the mnemonic.

Model: opus-5-5

internal/agekey/agekey_test.go

@@ -119,12 +119,12 @@ func TestAnExtraRecipientCanReadItTooAndSoCanTheDerivedOne(t *testing.T) {
 
 	mine, theirs := forIndex(t, 0), forIndex(t, 1)
 
+	to, err := agekey.Recipients([]string{theirs.Recipient()})
+	require.NoError(t, err)
+
 	var sealed bytes.Buffer
 
-	err := mine.Encrypt(
-		&sealed, strings.NewReader("hello"),
-		[]string{theirs.Recipient()}, false,
-	)
+	err = mine.Encrypt(&sealed, strings.NewReader("hello"), to, false)
 	require.NoError(t, err)
 
 	for _, key := range []*agekey.Key{mine, theirs} {
@@ -138,10 +138,7 @@ func TestAnExtraRecipientCanReadItTooAndSoCanTheDerivedOne(t *testing.T) {
 func TestARecipientThatIsNotOneIsRefused(t *testing.T) {
 	t.Parallel()
 
-	err := forIndex(t, 0).Encrypt(
-		&bytes.Buffer{}, strings.NewReader("hello"),
-		[]string{"not a recipient"}, false,
-	)
+	_, err := agekey.Recipients([]string{"not a recipient"})
 	require.Error(t, err)
 }