Conditional conformance for variadic parameters

[kyouko-taiga]

Sum and Tuple are effectively variadic generic types. It should be possible to declare a conditional conformance or extension based on a constraint defined over each generic parameter. That would allow, among other things:

• Make Sum<T1, ..., Tn> conform to ExpressibleBy***Literal when all its elements are; and
• Make Tuple<T1, ..., Tn> conform to Equatable when all its elements are.

These conformances may be synthesized before we support variadic parameters.