Feature request: "into"

I'd like to be able to initialize a `U256` like this:

```rust
foo(10.into());
```

Where `foo` is a function that takes a `U256` as an input.

This operation is not currently supported. I am getting the following error:

```text
help: the following other types implement trait `From`:
as From>
as From>
label: the trait `From<{integer}>` is not implemented for `Uint<256, 4>`
```