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).

To be pedantic: it's not factually wrong if the domain of discussion is e.g. the ends of certain I/O cables. (At least, I'm not aware of a new in vogue term there, though I am out of touch with current A/V terminology).

But otherwise I agree: it's trivial to change the example, and I would rather discussion not be further derailed.

Can we just assume for now that the author will adjust it in the near future?