Forbid partial outbound moves from immutable variables

This RFC proposes that the semantics of immutable variables be refined by forbidding partial outbound moves, so immutable variables in Rust become more immutable.

~~2. guaranteed lifetimes for values with move semantics ("movable values") can be achieved by combining this with explicit `drop` calls.~~

~~This makes `NoisyDrop`/`QuietDrop` from #210 unnecessary.~~

(EDIT: Guaranteed lifetimes can be provided by explicit `drop` calls alone.)

[Rendered View](https://github.com/CloudiDust/rfcs/blob/refine-immutables/active/0000-refine-immutables.md).