Introduce an "ExpressibleByCase" protocol (to facilitate struct-based enums with payload)

A couple more thoughts...

We could also provide an alternative that isn't stringly-typed:

/* Swift supplies init args by the position in which they are declared */
protocol ExpressibleByCaseIndex {
    init( `case`: Int, arguments: Any ... )

Maybe that's better in terms of performance, and prevention of typos.

Also, my preference is to allow cases to be usable with, or without their arguments: so we allow both .foo and .foo(123) and let the programmer decide how to handle that in init. If we want to give the programmer the option to allow/forbid this, maybe we could use a new syntax for declaring cases. Eg:

struct Param: ExpressibleByCase {
        height(Int)?,  // accepts .height(123)  *or*  .height
        length(Int)    // accepts .length(123)  *only* 

This would be the missing piece that makes a paradigm for accessing an enum payload (that the introduction of @dynamicCallable made possible), ergonomic.

Everyone has different tastes, but to mine, the struct paradigm is more convenient than Swift enums-with-payload.