Rollup merge of #125635 - fmease:mv-type-binding-assoc-item-constraint, r=compiler-errors

Rename HIR `TypeBinding` to `AssocItemConstraint` and related cleanup

Rename `hir::TypeBinding` and `ast::AssocConstraint` to `AssocItemConstraint` and update all items and locals using the old terminology.

Motivation: The terminology *type binding* is extremely outdated. "Type bindings" not only include constraints on associated *types* but also on associated *constants* (feature `associated_const_equality`) and on RPITITs of associated *functions* (feature `return_type_notation`). Hence the word *item* in the new name. Furthermore, the word *binding* commonly refers to a mapping from a binder/identifier to a "value" for some definition of "value". Its use in "type binding" made sense when equality constraints (e.g., `AssocTy = Ty`) were the only kind of associated item constraint. Nowadays however, we also have *associated type bounds* (e.g., `AssocTy: Bound`) for which the term *binding* doesn't make sense.

---

Old terminology (HIR, rustdoc):

```
`TypeBinding`: (associated) type binding
├── `Constraint`: associated type bound
└── `Equality`: (associated) equality constraint (?)
    ├── `Ty`: (associated) type binding
    └── `Const`: associated const equality (constraint)
```

Old terminology (AST, abbrev.):

```
`AssocConstraint`
├── `Bound`
└── `Equality`
    ├── `Ty`
    └── `Const`
```

New terminology (AST, HIR, rustdoc):

```
`AssocItemConstraint`: associated item constraint
├── `Bound`: associated type bound
└── `Equality`: associated item equality constraint OR associated item binding (for short)
    ├── `Ty`: associated type equality constraint OR associated type binding (for short)
    └── `Const`: associated const equality constraint OR associated const binding (for short)
```

r? compiler-errors

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