Remove async dependency - #255

Coverage decreased (-0.4%) to 95.918% when pulling aca3579 on mscdex:remove-async-dep into 158443d on form-data:master.

/cc @alexindigo

@mscdex Thanks for the PR. I do agree with the premise, async is too bloated. Although I don't think that 'form-data` is in business of flow control. We should remove code from here, not add.
And such things as parallel execution of async tasks should be in the separate properly tested library.

I replaced async with asynckit, it has zero dependencies and it provides more obvious API – where you iterate over list of values with iterator function, rather than executing list of the same function with different scope.

@alexindigo IMHO this single usage is simple enough and doesn't warrant an unnecessary dependency. However, the main goal is to dump async. I'm assuming when you said you replaced it, you did so locally, as I don't see the replacement in the master branch?

I created PR #258 and waiting for windows checks to pass. As soon as it's done. I'll merge and release new version. Thanks for the pushing it forward :)

PS. I wanted to replace iteration over functions with iteration over values. And even single use case requires tests, and tests for flow control in form-data lib, didn't feel like at home.

Published 2.0.0

Thanks for the quick responses!