[DSSv3] Fix Issue 13684 - std.numeric.isRectangular

[lucica28]

Added new function isRectangular

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	13684	enhancement	std.numeric.isRectangular

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8175"

[thewilsonator]

Retitle the commit message to say "Fix issue 13684 - add std.numeric.isRectangular"

[thewilsonator]

Needs documentation

[RazvanN7]

The commit message should contain the string "Fix Issue 13684 - ..." so that the bot picks it up. Upon merger, the bot will then automatically close the issue.

[kinke]

I don't think Phobos needs such a function; it may encourage people to think jagged arrays (arrays of arrays) are a reasonable way to represent such rectangular multidimensional arrays, while we have a much better suited library for that: http://mir-algorithm.libmir.org/mir_ndslice.html

Using join() for the recursion and thus requiring potentially huge temporary buffers for such a check is bad.

[RazvanN7]

@lucica28 As per @kinke 's comment I am going to close this PR. Feel free to close the issue as WONTFIX.