Recover parametricity using a matchable top type

[LPTK]

Currently, parametricity is violated because we can pattern-match on anything, including abstract types, thereby retrieving information that should not have been accessible.

We can easily fix this by requiring pattern matching scrutinees to be subtypes of some "matchable" upper bound which all concrete types subtype (but not type parameters).

This is similar to Scala 3's Matchable type (https://docs.scala-lang.org/scala3/reference/other-new-features/matchable.html).

[LPTK]

We're implementing this with the Object base class in #187.

[LPTK]

Note: Another advantage of Object is that it will allow us to cleanly separate primitive types, such as primitive integers, which have their own binary representation (in WASM), from object types. The two are distinct and should not be mixed in pattern matching expressions.