Specify initial data

[therustmonk]

Initial data on fisrt resuming is unsound and lost. It's important to take any piece data of resume send.

This PR fixes it, and makes possible to take value of first resume call.

It breaks backward compatibility, but not terrible.

@zonyitoo What's your opinion?

[zonyitoo]

Huh... why you need that? The closure itself can capture any initial data you want.

[therustmonk]

I send some fat data by pointer and restore it inside coroutine. But if I can't take first value, data will lost. I must to unpack it to prevent memory leaks, because use Box::into_raw().

Also I can't use one method and have to use two different like: resume_first() and resume_with(data) to resuming. I use array of Handles where coroutines can be in different states and I have to track order of resuming to use different methods. This adds unwanted overhead.

My coroutines are generic and created dynamically by user's code, that's why I need to send initial data sometimes.

[zonyitoo]

Hmm, sounds reasonable.
It just quite a little breaking change, which is very easy to modify. Merge it right now.

[therustmonk]

@zonyitoo I've checked it with my project, works fine! Thank you!