Switch to a Swift-friendly Base32 library

[mattrubin]

The current dependency for Base32 encoding and decoding exposes an Objective-C API which doesn't bridge nicely to Swift.

[mattrubin]

A prerequisite of this is probably converting the last of the legacy tests to Swift, since OTPTokenSerializationTests still uses the Objective-C Base32 interface.

[EmDee]

@mattrubin Anything to help out here? I noticed that the current Base32 dependency doesn't play nicely with Swift 5.3.

/cc @nrubin29

[mattrubin]

@EmDee Help is always welcome! Can you clarify what issues the current dependency has with Swift 5.3? I'm not seeing any issues with Xcode 12 beta 3.

[EmDee]

@mattrubin You're right. It was actually a problem on Carthage side (can't tell anymore, if it is due to missing Swift interface file, because the "Build for distribution" flag is not set, or because of this linker issue Carthage/Carthage#3019).