[Rust] 0.20.0 (Rust)

Added

• tee built-in command, mirroring the JavaScript implementation and GNU
  coreutils: -a/--append, -i/--ignore-interrupts, clustered short
  flags, -- as an option terminator, and a bare - treated as a file named
  -. A write failure is reported on stderr and sets exit code 1 while the
  remaining files are still written.
• Tests covering the StdinOption invariant that keeps stdio modes and input
  content in separate variants, so a mode can never be read as command input.