PNaCl support for Rust

[bvssvni]

https://mail.mozilla.org/pipermail/rust-dev/2014-July/010984.html

• Requires a custom version of Rust with cross compilation to PNaCl https://github.com/DiamondLovesYou/rust
• Rust PPAPI bindings https://github.com/DiamondLovesYou/rust-ppapi
• "Hello world!" with Rust PNaCl https://github.com/DiamondLovesYou/rust-pnacl-hello-world

[bvssvni]

If rust-lang/rust#16156 gets accepted, this might make it possible to use Rust nightly to build for custom target, like Emscripten and PNaCl.

rust-lang/rfcs#131

• RFC for the custom target