withUnsafeMutableBytes is now deprecated

[blackyhn]

Now in Swift 5 'withUnsafeMutableBytes' is deprecated.

var headerBuffer = [UInt8](repeating: 0, count: kCryptoExportImportManagerASNHeaderLengthForRSA);
headerBuffer[0] = kCryptoExportImportManagerASNHeaderSequenceMark;

[DigitalLeaves]

Hello, I have decided not to keep on upgrading Swift projects anymore. I have better things to do with my life than upgrading my 40+ projects every year :). Feel free to do a push request. I will review it and approve it, thanks!