[Feature] Unified type convert api of U256 and I256

### Component

primitives

### Describe the feature you would like

Now we could use `U256::to::()` and `I256::as_u64()`, but not vice visa.

In addition
* There's no `I256::as_u128`/`I256::as_i128`
* We have to use `f64::from(U256)`, but not `U256::to::()`
* There's even no `f64::from(I128)`

### Additional context

_No response_