decimal: replace homebuilt try_from with FromStr impl

https://github.com/exercism/rust/blob/9ad579f44024e459656d9646df9e8f0b33a3af13/exercises/decimal/example.rs#L30

It's more idiomatic and expressive to `impl FromStr for Decimal` instead of using a custom parse method. This is very slightly harder for the students, but it makes usage much nicer:

```diff
- let decimal = Decimal.try_from("123.45")?;
+ let decimal: Decimal = "123.45".parse()?;
```