decimal: replace homebuilt try_from with FromStr impl

[coriolinus]

rust/exercises/decimal/example.rs

Line 30 in 9ad579f

pub fn try_from(mut input: &str) -> Option<Decimal> {

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()?;