RFC: Export iszero

[ararslan]

As mentioned by @stevengj in #19947, introduced in #17623.

[tkelman]

if it's exported, it needs to be documented and the docs should be in the stdlib, if they aren't yet

[ararslan]

The web UI is cumbersome for this stuff, particularly for editing multiple files, so I'll improve this a little later. I can also add a testset for it.

[ararslan]

Now with 200% more docs and tests!

[ararslan]

Does this need a NEWS item?

[stevengj]

Yes, a NEWS item would be good.

[stevengj]

(I was thinking that iszero(A::Array{T}) could probably be made faster when T is one of the basic numeric types, since then you just need to search for a nonzero byte and could probably arrange it to use SIMD instructions on 256-bit chunks, but it's maybe not worth the trouble.)

[ararslan]

That would be pretty neat. I could take a crack at that at some point, but it would probably be better as a separate PR so that this and #19949 can be merged in time for 0.6.

[stevengj]

This assumes that people already know what this function is.

New `iszero(x)` function to quickly check whether `x` is zero (or is all zeros, for an array) ([#19950]).

[ararslan]

Much, much better. I've gone with that. Thanks!

[ararslan]

Actually, perhaps that optimization could be made in any/all, since the array-reducing method for iszero is just all(iszero, x).

[tkelman]

these types should probably just be written out - there's a type SOE.Sgnd defined in the core test that causes a failure here if it runs in the same worker