Revisions to SE-0366 from first round of review (#1775)

* Revisions to SE-0366 from first round of review

- `move` is renamed to `take`.
- Dropping a value without using it now requires an explicit
  `_ = take x` assignment again.
- "Movable bindings" are referred to as "bindings with static lifetime",
  since this term is useful and relevant to other language features.
- Additional "alternatives considered" raised during review
  and pitch discussions were added.
- Expansion of "related directions" section contextualizes the
  `take` operator among other planned features for selective copy
  control.
- Now that [ownership modifiers for parameters](https://forums.swift.org/t/borrow-and-take-parameter-ownership-modifiers/59581)
  are being pitched, this proposal ties into that one. Based on
  feedback during the first review, we have gone back to only allowing
  parameters to be used with the `take` operator if the parameter
  declaration is `take` or `inout`.

* Change motivating example to not entangle multiple variable lifetimes

As noted by @Jumhyn and others in evolution discussion, code motion optimizations
could still theoretically reorder operations in a way that breaks uniqueness that
relies on sequential execution to hold. Use an example that only requires forwarding
ownership of a single value to demonstrate the benefit of `take`.

* Add future direction discussing destructuring methods on move-only types.

It would be useful to be able to destructure move-only structs, but doing so
in the face of a `deinit` requires suppressing the deinit logic, since deinit
needs to start from a fully-initialized value to tear down. We could provide
a special operator for methods on such a type to do destructuring in limited
situations where it's appropriate for their API.

* Schedule SE-0366 for re-review.

Co-authored-by: Holly Borla <[email protected]>

Author: jckarter