Tracking Issue: Switch to upstream compiler

The goal is to eliminate any Redox-specific code in the Rust compiler so we don't need a fork.

• Finish removing old-format schemes
• Eliminate Scheme from Prefix in library/std/src/path.rs, and remove Prefix as an option for Path
• Review other commits from our fork and determine if any can be upstreamed or removed, or if something needs to be fixed, and add work items to this list
• Upgrade our fork to the latest Rust, with minimal difference from upstream
• Remove garbage "redox" code from the upstream (there are several items that are half-done or no longer needed)
• Push any changes from our fork to the upstream
• Update toolchain to use upstream