std: Add std.ArrayList(T).transform

[aloussase]

Transforming the data in an ArrayList is a common operation. I think it would be useful to have it as part of the standard library.

[aloussase]

Also as I was writing the PR I noted something with std.testing.expectEqual. The first argument is called expected but the test for std.ArrayList(u32).getLastOrNull() seems to be providing the expected value as the second argument, and the actual value as the first one.

In the tests I made for this PR I put the expected value as the first argument, but this required casting to u32 because comptime_int was being inferred. I think to fix this situation one of two things may be done:

1. Switch the order of arguments in std.testing.expectEqual
2. Add a type parameter to std.testing.expectEqual to avoid having to cast the expected value

I don't know if this is a real problem, but it is a little confusing seeing the expected value being provided in the position of the actual value.

[squeek502]

For the expectEqual stuff, see #4437 and #17431

[InKryption]

I'm failing to see why this:

list.transform(u32, &output_list, struct {
    fn transform(item: u32) u32 {
        return item + 1;
    }
});

would be preferable to this:

for (list.items) |item| {
    output_list.appendAssumeCapacity(item + 1);
}

or even better, to this:

try output_list.resize(list.items.len);
for (output_list.items, list.items) |*out, item| {
    out.* = item + 1;
}

Given the historical rejection of #940 and similar proposals, I believe this should be closed.