…nsequently switch to using `Array(Storage)` instead of `StorageArray`

- Introduce `alternatives` blocks as a way of asserting that a set of alternatives is exhaustive and mutually exclusive.
- Largely eliminate discussion of "substitution principle" for pattern matching as a whole, and instead focus on a "mirroring requirement" that applies to pattern functions specifically. Add extensive discussion of the potential drawbacks of this requirement.
- Add discussion of sum types that are implemented with sentinel values rather than discriminators.
- More comprehensive discussion of evaluation order

Co-authored-by: josh11b <[email protected]>

…so, explicitly discuss some issues that the OptionalPtr example was fudging, and mention the issue of `choice` alternative parameters having incomplete types.

…ing alternative. Also add discussion of binding patterns that mutate in-place.

…ion of "explicit inverses"

…move reference semantics to "open question" status.

Co-authored-by: josh11b <[email protected]>

…ove the discussion of bare designators to integrate better with it.

…oach.