Add library evolution support

[NathanJPhillips]

It should be noted that I'm not an expert so it may be something I'm doing wrong, but when I enable module stability in my library project that uses Sodium I get the following warning in my build logs. Is this something that needs to be enabled in the swift-sodium repository? If so, is there enough demand for this that it's worth somebody taking the time to do it?

warning: module 'Sodium' was not compiled with library evolution support; using it means binary compatibility for 'YourProjectName' can't be guaranteed

[johnalanwoods]

I don't think this is needed, per:

Library evolution support should only be used when a framework is going to be built and updated separately from its clients. In this scenario, a client built against an old version of the framework can be run with a new version of the framework without being recompiled.

[NathanJPhillips]

Not sure why that means it's not needed. When I build my framework that references Sodium it seems to want to build and include Sodium in the package too. Again, maybe that's what I'm doing wrongly but I didn't explicitly choose that behaviour.