What should we add?

This tracks the work labelled "PR 2" in the initial classical-expression design document.

Depends on #10223.

Brief overview:

• relax the type checking to permit WhileLoopOp.condition and IfElseOp.condition to be Expr values with type Bool, and SwitchCaseOp.target to be an Expr value with type Bool or Uint[n].

Closing this issue does not require adding support for the builder interface. That's tracked in a follow-up: #10228.