E.g. uint256.IntoBig(n *big.Int) that would essentially be the same as ToBig, just would use the provided sink instead of always allocating a new int. This could be needed for example in SSZ for encoding/decoding big.Int's going across uint256 for simplicity. Possibly I could manually roll big.Int without allocs too there, but it might be a good long term thing to have the conversion be alloc free.

The reason I suggested a new API is not to break ToBig, which is kind of useful in general, just not in alloc free cases.