decimal: replace homebuilt try_from with FromStr impl

[coriolinus]

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:

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