Rewrite the API to more closely bind libcurl

[alexcrichton]

These commits are a complete rewrite of the API of this crate based on binding more closely to the libcurl APIs themselves. This was originally started as a reimplementation to expose more features, but along the way I found that providing the same existing API may end up being more difficult than it's worth. In the end this rewrite performs a few things:

• All FFI bindings are now validated via ctest
• CI is now up and running for all platforms (including Windows platforms)
• Many many more underlying curl configuration options are exposed. There's also a clear path for exposing all the other ones!
• The path towards binding the curl multi interface is now much clearer, but not done just yet.

I'm gonna tag a bunch of issues and PRs as being closed by this, but I'd love to get some feedback on this as well! This PR currently requires the beta version of Rust as it uses the soon-to-be-stable std::panic APIs, so I won't make a release until those hit stable.

API docs

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