Not converting more than 64 bit number

I want to convert a 128 bit Hex number to binary using the Func bin().
But it seems to only support till 64 bit number.
Any help in this regard would be highly appreciated.

Start by locating the Func bin() implementation and its existing tests or examples, if present. Determine how the current 64-bit limit is enforced and define what successful conversion of a 128-bit hexadecimal value to binary should produce; done means that case converts correctly without regressing existing behavior.

Written by the indexing model from the issue text.