refactor: avoid extra owned string

I think this is more readable, too

src/formatting.rs

@@ -3,14 +3,14 @@ use rug::Float;
 use crate::nodes::{self, Error};
 
 pub fn pretty_eval(input: &str, max_digits: usize) -> Result<String, Error> {
-    let (prefix, input) = input
+    let (prefix, deliminator, input) = input
         .rsplit_once('=')
-        .map(|(p, i)| (p.to_owned() + "= ", i))
+        .map(|(p, i)| (p, "= ", i))
         .unwrap_or_else(|| {
             input
                 .rsplit_once('&')
-                .map(|(p, i)| (p.to_owned() + "& ", i))
-                .unwrap_or((String::new(), input))
+                .map(|(p, i)| (p, "& ", i))
+                .unwrap_or(("", "", input))
         });
 
     // no need to remove the character here,
@@ -33,7 +33,7 @@ pub fn pretty_eval(input: &str, max_digits: usize) -> Result<String, Error> {
             });
 
     Ok(format!(
-        "{prefix}{result:.display_digits$}{}",
+        "{prefix}{deliminator}{result:.display_digits$}{}",
         if should_add_back_backslash { " \\" } else { "" }
     )
     .replace('e', " * 10^"))