[P2] IO.exit, env-var set, and IO.args default handler

[ecto]

The IO effect is missing a few standard operations and one of the
existing ones has a surprising unhandled default.

• IO.exit — no way to terminate a program with a status code.
• env-var set — [IO.env "X"] reads; there is no setter.
• IO.args default — [IO.args] returns () (Unit) when run
  without a custom handler, which is hard to use. It should fall
  through to the actual process args as #[Str].

Acceptance criteria

• [IO.exit code], [IO.set-env "K" "V"] declared and listed in
  ref/effects.loon.
• Default IO.args handler returns the real process arguments.
• A test program reads [IO.args] under loon run and gets a
  non-empty vector.