Extended Try RFC

[Rendered view.](https://github.com/TyOverby/rfcs/blob/extended_try/active/0000-extended-try.md)
Given that the `try!` macros is widely used and mostly liked, creating a new macro seems like it would be a much better idea. In that case, I would recommend implementing it as an external library and getting some use and experience, then submitting a PR to add it to std (I don't think that would require an RFC), and perhaps finally an RFC to add it to the prelude if it is very popular.