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.go

@@ -94,15 +94,12 @@ func (k *Key) Identity() string {
 }
 
 // Encrypt copies src to dst, encrypted to this key and to every extra
-// recipient named, so the same mnemonic can always read it back again.
+// recipient given, so the same mnemonic can always read it back again.
 // Armored output is the text form age also reads.
 func (k *Key) Encrypt(
-	dst io.Writer, src io.Reader, to []string, armored bool,
+	dst io.Writer, src io.Reader, to []age.Recipient, armored bool,
 ) error {
-	all, err := recipients(k.identity.Recipient(), to)
-	if err != nil {
-		return err
-	}
+	all := append([]age.Recipient{k.identity.Recipient()}, to...)
 
 	out := dst
 
@@ -113,7 +110,7 @@ func (k *Key) Encrypt(
 		out = text
 	}
 
-	err = encrypt(out, src, all)
+	err := encrypt(out, src, all)
 	if err != nil {
 		return err
 	}
@@ -130,12 +127,12 @@ func (k *Key) Encrypt(
 	return nil
 }
 
-// recipients returns the key's own recipient followed by the ones
-// named on the command line, so what the key encrypts it can read.
-func recipients(mine age.Recipient, to []string) ([]age.Recipient, error) {
-	list := []age.Recipient{mine}
+// Recipients reads the recipients named on the command line, for
+// Encrypt to add to the key's own.
+func Recipients(names []string) ([]age.Recipient, error) {
+	list := make([]age.Recipient, 0, len(names))
 
-	for _, name := range to {
+	for _, name := range names {
 		parsed, err := age.ParseX25519Recipient(name)
 		if err != nil {
 			return nil, fmt.Errorf("reading recipient %q: %w", name, err)