age,cmd/age: add ParseRecipients and -R for recipient files

Fixes #84
Fixes #66
Closes #165
Closes #158
Closes #115
Closes #64
Closes #43
Closes #20

x25519.go

@@ -7,7 +7,6 @@
 package age
 
 import (
-	"bufio"
 	"crypto/rand"
 	"crypto/sha256"
 	"errors"
@@ -159,40 +158,6 @@ func ParseX25519Identity(s string) (*X25519Identity, error) {
 	return r, nil
 }
 
-// ParseIdentities parses a file with one or more private key encodings, one per
-// line. Empty lines and lines starting with "#" are ignored.
-//
-// This is the same syntax as the private key files accepted by the CLI, except
-// the CLI also accepts SSH private keys, which are not recommended for the
-// average application.
-//
-// Currently, all returned values are of type X25519Identity, but different
-// types might be returned in the future.
-func ParseIdentities(f io.Reader) ([]Identity, error) {
-	const privateKeySizeLimit = 1 << 24 // 16 MiB
-	var ids []Identity
-	scanner := bufio.NewScanner(io.LimitReader(f, privateKeySizeLimit))
-	var n int
-	for scanner.Scan() {
-		line := scanner.Text()
-		if strings.HasPrefix(line, "#") || line == "" {
-			continue
-		}
-		i, err := ParseX25519Identity(line)
-		if err != nil {
-			return nil, fmt.Errorf("error at line %d: %v", n, err)
-		}
-		ids = append(ids, i)
-	}
-	if err := scanner.Err(); err != nil {
-		return nil, fmt.Errorf("failed to read secret keys file: %v", err)
-	}
-	if len(ids) == 0 {
-		return nil, fmt.Errorf("no secret keys found")
-	}
-	return ids, nil
-}
-
 func (i *X25519Identity) Unwrap(block *Stanza) ([]byte, error) {
 	if block.Type != "X25519" {
 		return nil, ErrIncorrectIdentity