This PR addresses the issue where assignment validation would incorrectly pass when assigning an object to an interface type where methods had mismatched context parameters:

interface A {
    method(s: string): string;
}
interface B {
    method(this: void, s: string): string;
}
declare let a: A;
let b: B = a; // Should Error

Validation now recurses into interface and class types to check the methods being assigned. To avoid an infinite loop for recursive types, a cache is used to track previous comparisons. This has the fringe benefit of improving performance in code with a lot of assignments using the same types.

Additionally, the name of the function/method is now passed to the error when it occurs in an interface assignment. This is useful for tracking down the offending member in complex interface assignments.