[red-knot] Improve Symbol API for callable types (#14137)

## Summary

- Get rid of `Symbol::unwrap_or` (unclear semantics, not needed anymore)
- Introduce `Type::call_dunder`
- Emit new diagnostic for possibly-unbound `__iter__` methods
- Better diagnostics for callables with possibly-unbound /
possibly-non-callable `__call__` methods

part of: #14022 

closes #14016

## Test Plan

- Updated test for iterables with possibly-unbound `__iter__` methods.
- New tests for callables

crates/red_knot_python_semantic/resources/mdtest/call/function.md

@@ -44,3 +44,16 @@ reveal_type(bar())  # revealed: @Todo
 nonsense = 123
 x = nonsense()  # error: "Object of type `Literal[123]` is not callable"
 ```
+
+## Potentially unbound function
+
+```py
+def flag() -> bool: ...
+
+if flag():
+    def foo() -> int:
+        return 42
+
+# error: [possibly-unresolved-reference]
+reveal_type(foo())  # revealed: int
+```