Consider changing dev-lang/rust install paths

[wthrowe]

@brson has suggested changing Gentoo's installation layout in https://github.com/rust-lang/rust/pull/29878#issuecomment-157806399. I don't know what reasoning went into the current setup, so I'm just going to pass this information along.

[cardoe]

I followed the thread and I'm not really understanding his reasoning or logic behind his request. He acknowledges that we'll have to symlink things like man pages (for example but there are other files) and also acknowledges that its normal for _NIX based build systems to respect --_dir flags and even says they should work. But then says for Rust things should be special? There's a bunch of special casing in the rust installer scripts that could be simplified by following the traditional approaches. I've been having a terrible time trying to get Rust working nicely inside of Yocto and as a Gentoo developer I've been looking to this repo for ideas.