Promise:await() method

[Joakker]

Is your feature request related to a problem? Please describe.
Javascript has a promise system similar to the one offered by this crate. However, it also comes with a big problem called Callback Hell, which basically looks like this:

The solution JS opted for is to introduce the await keyword. Since this crate is supposed to be language agnostic, it should instead add a method for that functionality.

This is important because, if you want to use more than one value from rust at a time, you would have to nest the callbacks, like so:

get_my_value1():and_then(function(value1)
	get_my_value2():and_then(function(value2)
		-- etc.
	end)
end)

Describe the solution you'd like

With the :await() method, the above code should look like this. Much nicer:

local value1 = get_my_value1()
	:and_then(function(value)
		return value
	end)
	:await()

local value2 = get_my_value2()
	:and_then(function(value)
		return value
	end)
	:await()

--- etc

The await method should wait until the callback function returns, and then give back the final value.

Additional context
Nested callbacks look ugly