Pre-RFC: Cargo profile setting to sanitise host-dependent absolute paths, enabled by default for release builds

[zackw]

@cbeuw

• Cargo's home directory will be remapped to /cargo on Unix and \\cargo on Windows
• Rust's sysroot will be remapped to /sysroot on Unix and \\sysroot on Windows

@bill_myers

I'm not sure that "." is the best prefix for the current project though, maybe something like "//rust/work/[crate name]/[git tree hash for current directory contents]" might be better

Please let's not make up top level directories or UNC paths (what if there exists a CIFS server addressable as \\sysroot on someone's LAN?) @CAD97's suggestion of beginning all the remappings with $CARGO_SOMETHING_OR_OTHER/ seems safer to me.