v1.4.0-preview.1

Changes:

• Updated libsodium to 1.0.19.2
• Added support for iOS, tvOS, and MacCatalyst

Note that this has not been extensively tested. Please provide feedback in #80 whether it works or not.

Please indicate the target platform (iOS, tvOS, or MacCatalyst), development platform (macOS, Windows, ...), and .NET SDK used, including version numbers.

Both positive and negative reports are welcome!