feat: v2

[peterboyer]

• redesigns ADT<> as primary way to define ADT variant types
  • requires that you use a union of ADT<...> | ADT<...> | ...
  • allows that many variants with the same data type can be defined without code duplication
• redesigns ADT<T>() as primary way to construct variant values
  • build ADT types first, then pass as T type parameter.
  • returns shared memory-efficient proxy to create values
  • constructors are shared based on type
• removes all ADT namespace utilities
  • Keys can be derived from X['$type'].
  • Pick can be derived from Extract<X, ADT<"PickThis">>.
    • Equivalent to Extract<X, { $type: "PickThis" }>.
  • Omit can be derived from Exclude<X, ADT<"OmitThis">>.
    • Equivalent to Exclude<X, { $type: "OmitThis" }>.
• removes custom discriminant support
  • much easier to cast an external ADT than introduce broad interface complexity to support custom discriminants