Make word list const

This makes the pointers to the words constant.  It moves 8kb from ram
to flash.  It changes the return type of mnemonic_wordlist() to reflect
this change.  Everyone calling it should also change the type to
`const char * const *`.

bip39.c

@@ -155,7 +155,7 @@ void mnemonic_to_seed(const char *mnemonic, const char *passphrase, uint8_t seed
 	pbkdf2_hmac_sha512((const uint8_t *)mnemonic, strlen(mnemonic), salt, saltlen, BIP39_PBKDF2_ROUNDS, seed, 512 / 8, progress_callback);
 }
 
-const char **mnemonic_wordlist(void)
+const char * const *mnemonic_wordlist(void)
 {
 	return wordlist;
 }

bip39.h

@@ -36,6 +36,6 @@ int mnemonic_check(const char *mnemonic);
 
 void mnemonic_to_seed(const char *mnemonic, const char *passphrase, uint8_t seed[512 / 8], void (*progress_callback)(uint32_t current, uint32_t total));
 
-const char **mnemonic_wordlist(void);
+const char * const *mnemonic_wordlist(void);
 
 #endif

bip39_english.h

@@ -21,7 +21,7 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
-const char *wordlist[] = {
+const char * const wordlist[] = {
 "abandon",
 "ability",
 "able",