Implement `From<T>` for `BigInt`/`BigUint` for signed and unsigned integer types (respectively)

[hauleth]

As in title. As any integer type can be expressed as BigInt and any unsigned integer can be expressed as BigUint it would be useful to implement From<T> trait for them. It would allow functions to take T: Into<BigInt> as parameter and call it with "bare" integers without weird calls and unwraps.