Narrow pkg/cbk to keypair derivation

Remove KeyFromHex, Seal, and Open with their errors and constants. The
library only derives crypto_box keypairs from BIP39 mnemonics; callers
use golang.org/x/crypto/nacl/box directly for everything else.

README.md

@@ -67,29 +67,14 @@ func main() {
     }
 
     // The same mnemonic (and optional passphrase) always yields the same
-    // keypair, on any machine.
-    alice, err := cbk.New(cbk.Params{Mnemonic: mnemonic})
+    // keypair, on any machine. The keys are plain crypto_box keys for use
+    // with golang.org/x/crypto/nacl/box.
+    keypair, err := cbk.New(cbk.Params{Mnemonic: mnemonic})
     if err != nil {
         panic(err)
     }
 
-    bob, err := cbk.FromMnemonic("abandon abandon abandon abandon abandon " +
-        "abandon abandon abandon abandon abandon abandon about")
-    if err != nil {
-        panic(err)
-    }
-
-    sealed, err := alice.Seal([]byte("hello bob"), bob.PublicKey)
-    if err != nil {
-        panic(err)
-    }
-
-    plain, err := bob.Open(sealed, alice.PublicKey)
-    if err != nil {
-        panic(err)
-    }
-
-    fmt.Println(string(plain), alice.PublicKeyHex())
+    fmt.Println(keypair.PublicKeyHex())
 }
 ```
 
@@ -143,9 +128,9 @@ directory is never read.
 
 ### `pkg/cbk`
 
-Derives NaCl crypto_box keypairs (Curve25519, XSalsa20, Poly1305)
-deterministically from BIP39 mnemonics, and seals and opens messages with them.
-The derivation is fixed and documented so keys are reproducible:
+Derives NaCl crypto_box keypairs (Curve25519) deterministically from BIP39
+mnemonics, and nothing else. The derivation is fixed and documented so keys are
+reproducible:
 
 1. The mnemonic is validated and canonicalized with
    `bip39wordlist.NormalizeWordlist`.
@@ -156,16 +141,13 @@ The derivation is fixed and documented so keys are reproducible:
    `HKDF-SHA256(seed, salt = nil, info = cbk.DerivationInfo)`.
 4. The public key is the Curve25519 base-point multiple of the private key.
 
-A sealed message is a 24-byte random nonce followed by the box ciphertext, so it
-can be handled as a single opaque byte string. The API:
+The resulting keys are plain 32-byte crypto_box keys for use with
+`golang.org/x/crypto/nacl/box`. The API:
 
 - `cbk.New(cbk.Params{Mnemonic, Passphrase})` and `cbk.FromMnemonic(m)` return a
   `CryptoBoxKeypair`.
-- `CryptoBoxKeypair.Seal(message, recipientPublicKey)` and
-  `CryptoBoxKeypair.Open(sealed, senderPublicKey)`.
-- `CryptoBoxKey.Hex()`, `CryptoBoxKey.Bytes()`, `cbk.KeyFromHex(s)`, and the
-  `PublicKeyHex`, `PrivateKeyHex`, `PublicKeyBytes`, `PrivateKeyBytes` accessors
-  on the keypair.
+- `CryptoBoxKey.Hex()`, `CryptoBoxKey.Bytes()`, and the `PublicKeyHex`,
+  `PrivateKeyHex`, `PublicKeyBytes`, `PrivateKeyBytes` accessors on the keypair.
 
 The test suite pins the keypair derived from the standard all-zero-entropy BIP39
 test vector. If that test ever fails, the derivation has changed and existing