feat[expr]: new fill null expr

[joseph-isaacs]

Creates an expression that replaces null values with a fill value.

# use vortex_array::expr::{fill_null, root, lit};
let expr = fill_null(root(), lit(0i32));

[codspeed-hq]

Merging this PR will improve performance by 33.74%

⚡ 1 improved benchmark
✅ 1137 untouched benchmarks
⏩ 1265 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	chunked_bool_into_canonical[(1000, 10)]	87.5 µs	65.4 µs	+33.74%

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_{Comparing ji/fill-null-expr (edcd15f) with develop (954311f)}

Footnotes

1. 1265 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩