1. Add `BlockCreator.switch_()` and `switchEnum()` overloads that accept
   `Class` as the `switch` expression type, in addition to `ClassDesc`.
   Often, the type is statically known and easier to express as `Class`.
2. Add validation that at least one `case` or `default` branch is added
   to a `switch`. If none is added, an exception is thrown early.
3. Allow hash-based `switch` implementations to have no `case` branches,
   only a `default` branch. This condition occurs in practice when `case`
   branches are generated based on an external list of objects. Such lists
   may easily be empty. This has always been allowed for value-based `switch`
   implementations (called "perfect hash" based here).
4. Let `switch` expressions (`switch` with a non-`void` type) always fall
   through. This allows (and, in fact, requires) using them as actual
   expressions.

   Note that `switch` statements (`switch` with a `void` type) behave as
   before: if no branch may fall through, the entire `switch` may not either.

Tests are added for items 3 and 4.