Richer So() assertions

[gdamore]

We could implement an extended version of So that is variadic, and then define custom assertions against them, like GoConvey has. I imagine that we'd have an "assertion" type that's actually a function pointer (could be variadic), that returns a result, which would include information appropriate for logs, and which would take the arguments.

Because C is strongly typed, we'd probably need versions of these macros that understand different types. I think this would be relatively straight-forward, other than it requires C99 variadics. (Fortunately I think these are supported ~everywhere these days.)