Support macOS 10.15, iOS 13, tvOS 13, and watchOS 6

[stackotter]

I would like to use swift-png on macOS versions earlier than macOS 13.3, and afaict swift-hash is the only reason that swift-png requires macOS 13.3+. The changes to support macOS versions back to 10.15 were luckily pretty minimal. While I was at it I also added the corresponding iOS, tvOS, etc versions to my availability checks (but I haven't tested compilation on those platforms, only on macOS).

Changes

• Added @available annotations to ExpressibleByIntegerLiteral implementations that rely on StaticBigInt
• Used #available to provide an alternative CustomStringConvertible implementation for InlineBuffer when String.init(unsafeUninitializedCapacity:initializingUTF8With:) isn't available
• Limited MD5Tests to macOS 13.3+ since those tests rely heavily on being able to express MD5 using integer literals. This was achieved using a runtime check because I couldn't find a way to make it a compile-time check (there doesn't seem to be compile-time conditional that allows checking platform versions), but if you know a way I'd be happy to update my implementation.

The alternative implementation for InlineBuffer.description probably isn't as efficient as the post macOS 11 implementation unless the compiler does something clever, but given that the older platforms weren't supported at all before I reckon this is an ok compromise (and swift-png doesn't rely on that part of the code 😅).

[tayloraswift]

thanks! 💝