feat(data/nat/cast): add nat.bin_cast for faster casting (#5664)
feat(data/nat/cast): add nat.bin_cast for faster casting (#5664)

[As suggested](https://github.com/leanprover-community/mathlib/pull/5462#discussion_r553226279) by @gebner.



Co-authored-by: Yakov Pechersky <9925c0c20eafe10b1eaac37c5fe124dc0b28ca8c@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib