Simplify metasync syntax

[tshemsedinov]

Now flow syntax is:

const f1 = (c, cb) => cb();
const f2 = (c, cb) => cb();
const f3 = (c, cb) => cb();
const f4 = (c, cb) => cb();
const fc = metasync([f1, f2, f3, f4]);
fc({}, (err, context) => {});

It will be good to have simplified syntax:

const f1 = cb => cb();
const f2 = cb => cb();
const f3 = cb => cb();
const f4 = cb => cb();
const fc = metasync([f1, f2, f3, f4]);
fc((err, context) => {});

Here are two optimizations:

• composing function may have no context argument
• we may call composed function with no context argument

[tshemsedinov]

Second case done, first case isn't implemented because it can affect performance.