Add support for sigma types

[alexchandel]

A sigma 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.

/**
 * @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!).