finish as_ref_mut exercise

exercises/conversions/as_ref_mut.rs

@@ -2,16 +2,15 @@
 // Read more about them at https://doc.rust-lang.org/std/convert/trait.AsRef.html
 // and https://doc.rust-lang.org/std/convert/trait.AsMut.html, respectively.
 
-// I AM NOT DONE
 // Obtain the number of bytes (not characters) in the given argument
 // Add the AsRef trait appropriately as a trait bound
-fn byte_counter<T>(arg: T) -> usize {
+fn byte_counter<T: AsRef<str>>(arg: T) -> usize {
     arg.as_ref().as_bytes().len()
 }
 
 // Obtain the number of characters (not bytes) in the given argument
 // Add the AsRef trait appropriately as a trait bound
-fn char_counter<T>(arg: T) -> usize {
+fn char_counter<T: AsRef<str>>(arg: T) -> usize {
     arg.as_ref().chars().count()
 }