Note: This is a reland of #84035.
The standard specifies that it it UB to specialize the following traits:
- `std::is_integral`
- `std::is_unsigned`
- `std::make_unsigned`
- `std::make_signed`
This patch:
- Removes specializations for `BigInt`
- Transforms SFINAE for `bit.h` functions from template parameter to
return type (This makes specialization easier).
- Adds `BigInt` specialization for `bit.h` functions.
- Fixes code depending on previous specializations.
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