Refactors the `Bindings` structure to use a two-level representation:
- Outer level: union elements (each can be a single callable or an intersection)
- Inner level: bindings within an intersection element
This enables proper handling of types like `(A & B) | C` when calling,
where `A & B` is an intersection that was narrowed by `callable()`.
Key changes:
- Add `BindingsElement` struct to represent a single union element
(which may contain multiple bindings for intersections)
- Update `from_union` to preserve intersection structure instead of
flattening
- Update `from_intersection` to create a single element with multiple
bindings
- Update `return_type`, `check_types_impl`, and `report_diagnostics`
to handle the two-level structure
- Add test for union-of-intersections case
The priority hierarchy for intersection call errors is preserved: when
all bindings in an intersection fail, only the highest-priority error
type is reported (BindingError > TopCallable > NotCallable).
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