Tuple <-> Array conversion helpers

Proposal

Problem statement

Transferring tuples into arrays is unwieldy where necessary.

Motivating examples or use cases

https://docs.rs/nalgebra/latest/nalgebra/base/struct.Matrix.html#method.swap

<[_; 2]>::from(x);
<(_, _)>::from(x);

Solution sketch

impl<T> (T, T…) {
   fn array(self) -> [T; len((T, T…))];
}
impl<T, const N: usize> [T; N] {
   fn tuple(self) -> (T, n…);
}

https://docs.rs/atools/latest/atools/trait.Array.html

Alternatives

Do nothing.

Links and related work

#847

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start by reading related issue #847 and the Rust library feature lifecycle linked in the issue. No source file or test entry point is named; done currently means developing and documenting a sufficiently supported API proposal for libs-api team review, rather than implementing a defined change.