Adopt rust-bindgen

[graydon]

https://github.com/crabtw/rust-bindgen is super cool and ought to be part of the toolset we distribute with rustc.

It might be the right time to consider bundling libclang with rustc as well, for this reason as well as possibly other forms of integration (our driver, cargo, etc.)

[graydon]

Link with #2238

[z0w0]

Closing this because it's a duplicate of #2124 which has more detail about how the frontend would work.