[red-knot] use declared types in inference/checking (#13335)

Use declared types in inference and checking. This means several things:

* Imports prefer declarations over inference, when declarations are
available.
* When we encounter a binding, we check that the bound value's inferred
type is assignable to the live declarations of the bound symbol, if any.
* When we encounter a declaration, we check that the declared type is
assignable from the inferred type of the symbol from previous bindings,
if any.
* When we encounter a binding+declaration, we check that the inferred
type of the bound value is assignable to the declared type.

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -27,7 +27,9 @@ pub mod expression;
 pub mod symbol;
 mod use_def;
 
-pub(crate) use self::use_def::{BindingWithConstraints, BindingWithConstraintsIterator};
+pub(crate) use self::use_def::{
+    BindingWithConstraints, BindingWithConstraintsIterator, DeclarationsIterator,
+};
 
 type SymbolMap = hashbrown::HashMap<ScopedSymbolId, (), ()>;