Add support for sigma types

A [sigma type](https://en.wikipedia.org/wiki/Dependent_type) or "dependent pair" is a type determined by the value of another variable. A common example is C++ templates with `int` parameters.

For example, consider the contrived case below, except where `b` is a `number` if `a == Result.PRIME`, but an `Array` if and only if `a == Result.COMPOSITE`.

```js
/**
* @enum
*/
Result = {
PRIME: 1,
COMPOSITE: 0
}

/**
* @param {Result} a
* @param {number|Array<[number, number]>}
*/
function foo(a, b) {
// ...
}
```

At the moment, this cannot be annotated, and it requires manual testing and `/** @type {SomeClass} */` casting. But the compiler has the ability to reason about valid values, and validate these checks for you (free of human error!).