perf: Optimize scalar path for ascii function

[kumarUjjawal]

Which issue does this PR close?

• Part of [EPIC] Optimize performance for slow expressions datafusion-comet#2986.

Rationale for this change

The ascii function currently converts scalar inputs to arrays before processing via make_scalar_function. Adding a scalar fast path avoids this overhead and improves performance.

What changes are included in this PR?

1. Refactored invoke_with_args to use match statement for handling both scalar and array inputs
2. Added scalar fast path for Utf8, LargeUtf8, and Utf8View scalar inputs

Type	Before	After	Speedup
ascii/scalar_utf8	234 ns	56 ns	4.2x
ascii/scalar_utf8view	206 ns	57 ns	3.6x

Are these changes tested?

Yes

Are there any user-facing changes?

No

[alamb]

Nice work @kumarUjjawal

BTW if you wanted to optimize this more another thing you can do is to make two different loops -- when there is null and when there are no nulls, rather than checking on each iteration here

datafusion/datafusion/functions/src/string/ascii.rs

Lines 102 to 120 in 998f534

	fn calculate_ascii<'a, V>(array: &V) -> Result<ArrayRef, ArrowError>
	where
	V: StringArrayType<'a, Item = &'a str>,
	{
	let values: Vec<_> = (0..array.len())
	.map(|i| {
	if array.is_null(i) {
	0
	} else {
	let s = array.value(i);
	s.chars().next().map_or(0, |c| c as i32)
	}
	})
	.collect();
	let array = Int32Array::new(values.into(), array.nulls().cloned());
	Ok(Arc::new(array))
	}