crypto-square: replace repeated push with resize (#1025)

clippy::same_item_push
https://rust-lang.github.io/rust-clippy/master/index.html#same_item_push

Helps address #1012

Author: petertseng

exercises/crypto-square/example.rs

@@ -47,10 +47,7 @@ fn prepare(input: &str) -> Vec<char> {
     // add space padding to the end such that the actual string returned
     // forms a perfect rectangle
     let (r, c) = dimensions(output.len());
-    let padding_qty = (r * c) - output.len();
-    for _ in 0..padding_qty {
-        output.push(' ');
-    }
+    output.resize(r * c, ' ');
 
     output.shrink_to_fit();