Add swift implementation

Pure Swift implementation without any third-party dependencies。

Author: pengpengliu

bip-0039.mediawiki

@@ -165,6 +165,7 @@ Rust:
 Swift:
 * https://github.com/CikeQiu/CKMnemonic
 * https://github.com/yuzushioh/WalletKit
+* https://github.com/pengpengliu/BIP39
 
 C++:
 * https://github.com/libbitcoin/libbitcoin-system/blob/master/include/bitcoin/system/wallet/mnemonic.hpp