Better display options support 

It's wonderful that we're [able](https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.precision) to take fractional numeric formatting options for convenient formatting syntax.

```rust
let x: FloatingPoint<i128, U18>;
format!("{0:.5}", x)
```

Let's support that stuff!

Probably it willl be useful to pass through single named formatting option to provide serialization strategy.