UintXX:full_mul consistency

[webmaster128]

Uint{64, 128, 256}::full_mul should all take Into<Self> as the right hand side. The reason is that mixed type arthmetic operations are discouraged. When we get Uint64*Uint64 -> Uint128, Uint128*Uint128 -> Uint256 and Uint256*Uint256 -> Uint512 where * denotes the full multipleciation operation. Keeping Into<Self> instead of just Self makes it convenient to work with and non-breaking in the vast majority of cases.

Then we can have pretty much the same implementation for all 3 cases:

    pub fn full_mul(self, rhs: impl Into<Self>) -> Uint128 {
        let a = Uint128::from(self);
        let b = Uint128::from(rhs.into());
        a.checked_mul(b).unwrap()
    }