evaluator: take advantage of disjunction elements being mutually exclusive

[myitcv]

This is a placeholder issue for performance improvements in the new evaluator that take advantage of elements of a disjunction being mutually exclusive. This can happen in a number of situations:

• Mutually exclusive concrete values: 1 | 2 | 3
• Elements that are mutually exclusive by virtue of their type: int | string | {}
• Structured values that have elements which are mutually exclusive, otherwise referred to as discriminator fields: {kind: "k1"} | {kind: "k2"}

The new evaluator will enable a vast performance improvement in the resolution of such disjunctions, as well as provide better error messages.

(More detail required here for the use cases that will be "covered" by this issue).