Extract Payload for enum cases having associated value

[Jumhyn (Frederick Kellison-Linn)]

Handling the shared base names situation seems like something that could fall out of an implementation of the "next steps" after SE-0111:

@Chris_Lattner

...extend declaration names for variables, properties, and parameters to allow parameter names as part of their declaration name. For example:

var op(lhs:,rhs:) : (Int, Int) -> Int // variable or property.
x = op(lhs: 1, rhs: 2) // use of the variable or property.

// API name of parameter is “opToUse”, internal name is "op(lhs:,rhs:)”.
func foo(opToUse op(lhs:,rhs:) : (Int, Int) -> Int) {
    x = op(lhs: 1, rhs: 2) // use of the parameter
}
foo(opToUse: +) // call of the function

Where these compound names could also be used to refer to enum cases as, e.g. syntaxTree[keyPath: \.type(variables:)].