Forbid partial outbound moves from immutable variables

This RFC proposes that the semantics of immutable variables be refined by forbidding partial outbound moves, so:

1. 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.

[Rendered View](https://github.com/CloudiDust/rfcs/blob/refine-immutables/active/0000-refine-immutables.md).

Rust' score values include inclusivity. Please refer to the Code of Conduct.

Beyond that, this is just factually wrong. Intersex people exist.

Furthermore, it is trivial to provide another example that isn't exclusionary.=