Refactor: move redox-exec into redox-rt::proc.

Cargo.lock

Cargo.toml

redox-rt/Cargo.toml

redox-rt/src/arch/aarch64.rs

redox-rt/src/arch/mod.rs

redox-rt/src/arch/x86.rs

redox-rt/src/arch/x86_64.rs

redox-rt/src/lib.rs

redox-rt/src/proc.rs

src/platform/mod.rs

src/platform/redox/clone.rs

src/platform/redox/exec.rs

src/platform/redox/extra.rs

src/platform/redox/mod.rs