This PR adds type checking for class expressions, finishing the work started in #2567.

In a class expression, the class name is optional and, if specified, is only in scope in the class expression itself. This is similar to the optional name of a function expression. It is not possible to refer to the class instance type of a class expression outside the class expression, but the type can of course be matched structurally.

Class declarations and class expressions differ as follows:

• A class declaration of the form class C { } introduces an instance type named C and a constructor function named C with the type new () => C.
• A class expression of the form class C { } produces a constructor function value of type new () => C and introduces an instance type named C that is in scope only within the body of the class expression.
• A class expression of the form class { } produces a constructor function value of type new () => { ... } where { ... } is the anonymous instance type of the class expression.

An example:

let Point = class {
    constructor(public x: number, public y: number) { }
    public length() {
        return Math.sqrt(this.x * this.x + this.y * this.y);
    }
};
var p = new Point(3, 4);  // p has anonymous class type
console.log(p.length());

Fixes #497.