`BigInt` and `Int128` are incompatible

There’s no good reason why you shouldn’t be able to initialize a `BigInt` with a `Int128`.
```d
void main()
{
    import std.int128;
    import std.bigint;
    Int128 x;
    BigInt y = x; // error
    BigInt z = x.data.hi * (BigInt(1L) << 64) + x.data.lo;
}
```