This PR improves contextual typing by generic rest parameters such that we can infer a tuple type from the function expression or arrow function being contextually typed while simultaneously assigning contextual parameter types to the function expression or arrow function. Consider:

declare function pipe<A extends any[], B, C>(f: (...args: A) => B, g: (x: B) => C): (...args: A) => C;

let x1 = pipe(() => true, b => !b);  // () => boolean
let x2 = pipe(x => "hello", s => s.length);  // (x: any) => number
let x3 = pipe((x, y) => ({ x, y }), o => o.x === o.y);  // (x: any, y: any) => boolean
let x4 = pipe((x: number) => x * x, x => "" + x);  // (x: number) => string

In the examples above, the type inferred for A is [], [any], [any, any], and [number] respectively. Previously, in all but the last example we'd infer any[] for A.