Improve performance of `JArray`'s `operator []=`

[HosseinYousefi]

Unrelated to this PR

AI for me: benchmark if allocating on heap every time just to do []= is more expensive than creating an accessor function that uses the pointer to a variable on the stack to set the array region.

Originally posted by @HosseinYousefi in #1095 (comment)

[HosseinYousefi]

Benchmark here: https://github.com/HosseinYousefi/jnigen_benchmark/tree/array

So it makes sense to do this.