decimal: use ? instead of try_opt

Since 1.22, have been able to use ? for Option.
https://doc.rust-lang.org/edition-guide/rust-2018/error-handling-and-panics/the-question-mark-operator-for-easier-error-handling.html

Co-authored-by: Peter Goodspeed-Niklaus <[email protected]>

Author: petertseng

exercises/decimal/Cargo-example.toml

@@ -4,6 +4,5 @@ name = "decimal"
 version = "0.1.0"
 
 [dependencies]
-try_opt = "0.1.1"
 num-bigint = "0.1.40"
 num-traits = "0.1.40"

exercises/decimal/example.rs

@@ -2,9 +2,6 @@ use std::cmp::Ordering;
 use std::fmt;
 use std::ops::{Add, Mul, Sub};
 
-#[macro_use]
-extern crate try_opt;
-
 extern crate num_bigint;
 use num_bigint::BigInt;
 extern crate num_traits;
@@ -55,7 +52,7 @@ impl Decimal {
             }
         }
         Some(Decimal::new(
-            try_opt!(digits.parse::<BigInt>().ok()),
+            digits.parse().ok()?,
             decimal_index.unwrap_or_default(),
         ))
     }