Add surface syntax for "this type" (#1236)

Within the context of an aggregate type (or an `extension` of one), the programmer can use `this` to refer to the "current" instance of the surrounding type, but there is no easy way to utter the name of the type itself. This is especially relevant inside of an `interface`, where the type of `this` isn't actually the `interface` type, but rather a placeholder for the as-yet-unknown concrete type that will implement the interface.

This change adds a keyword `This` that works similarly to `this`, but names the current *type* instead of the current instance. It can be used to declare things like binary methods or factory functions in an interface:

```
interface IBasicMathType
{
    This absoluteValue();
    This sumWith(This left);
}

T doSomeMath<T:IBasicMathType>(T value)
{
    return value.sumWith(value.absoluteValue());
}
```

The `This` type is consistent with the type named `Self` in Rust and Swift (where Rust/Swift use `self` instead of `this`). Other names could be considered (e.g., `ThisType`) if we find that users don't like the name in this change.

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