[red-knot] Rename constraint to predicate

[dcreager]

In #16306 (comment), @carljm pointed out that #16306 introduced a terminology problem, with too many things called a "constraint". This is a follow-up PR that renames Constraint to Predicate to hopefully clear things up a bit. So now we have that:

• a predicate is a Python expression that might influence type inference
• a narrowing constraint is a list of predicates that constraint the type of a binding that is visible at a use
• a visibility constraint is a ternary formula of predicates that define whether a binding is visible or a statement is reachable

This is a pure renaming, with no behavioral changes.

[carljm]

Thank you!