add mapv_into_any(), resolves #1031

[benkay86]

First attempt at pull request for #1031.

Since Rust does not yet have specialization, mapping between generic numerical types in ndarray is challenging when the types might be different. For example, we might want to perform on operation on an array of real numbers to get an array that might be real or might be complex. We can do this now with ArrayBase::mapv(), but if it turns out that both the input and output are the same type (e.g. real numbers) then mapv() will perform unnecessary allocation of a new array compared to mapv_into().

This PR proposes a generic method ArrayBase::mapv_into_any() whose signature accepts a closure mapping between different types FnMut(A) -> B but which uses memory-efficient mapv_into() when A and B are the same type. The type comparison is performed at runtime. This is the most memory-efficient way to map between arrays of possibly different types using stable Rust.

The use of unsafe is discussed on the Rust language forum.

[benkay86]

Thank you for granting permission to run git workflows on this pull request. It looks like clippy on the beta channel rejects a large portion of the ndarray codebase, but it appears that this pull request has passed all checks.

[bluss]

It just looks like clippy found new lints in a new release

[bluss]

Looks solid to me, should be useful

[bluss]

Please use issue closing keywords in the pr description so that the linked issue is closed when this is merged https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue

[benkay86]

Fixed commit comment.

Edit: Looks like you'll still need to manually link to issue #1031, sorry. I don't appear to have permission.

[bluss]

FWIW, we already use this kind of ad-hoc specialization in ndarray with the linalg code, so it feels familiar to us.

[bluss]

(milestone - not known if we will have 0.15.4 or 0.16 atm). Thanks!