`Promise:await()` method

**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:

```lua
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:

```lua
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

The issue names no files, tests, or entry points. Start by locating the crate's Promise implementation and its Lua-facing API, then determine how callback completion is currently represented. Done would require a defined Promise:await() behavior that supports the shown usage and corresponding tests.

Written by the indexing model from the issue text.