Inline is_valid calls in PrimitiveIter/BooleanIter

[jhorstmann]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Shortly discussed in #1048 (comment)

The calls to is_valid do not seem to get inlined in PrimitiveIter and some other places, leading to slower than optimal performance.

Describe the solution you'd like

Mark ArrayData::is_valid/is_null as inline and also add is_valid_unchecked methods for use in trusted code.

Could also try using a ScalarBuffer from #1825 and a separate bitmap instead of a PrimitiveArray field in the iterator.

Describe alternatives you've considered

Additional context

Ideally I'd like code like the following the be completly inlined and basically be equivalent to a memcpy:

fn slice_from_array(a: &Int32Array, output: &mut[i32]) {
    output.iter_mut().zip(a.iter()).for_each(|(out, x)| {
        *out = x.unwrap_or(0);
    });

That would require the compiler to move the check for presence of a null buffer outside of the loop and generate two optimized loops, one for nullable and one for non-null arrays.

[jhorstmann]

In a simplified example, llvm is able to completely unswitch a loop based on the presence of a validity bitmap, and generate two separately optimized versions of a kernel: https://rust.godbolt.org/z/sxhsh3h7G