arrow-buffer: make i256::leading_zeros public

[theirix]

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

Function leading_zeros exists for other integer types, and for a wide i128 as well. It's worth having it as a public part of the struct.

Describe the solution you'd like

Making it a supported public function

Describe alternatives you've considered

Coding it as an external function.

Additional context

Required particularly for DataFusion UDF implementations