Add append to strings.

README.md

@@ -777,6 +777,7 @@ The following standard functions (defined in the standard library but excluded i
 
 * `len` - returns the number of characters (not number of bytes) in the string
 * `pad` - pads the string with an character until a specified number of characters
+* `append` - Adds a character or a string to the end of another string
 * `clear` - empties the string
 * `truncate` - cuts off the string at exactly a specified number of characters
 * `contains` - checks if a certain character or sub-string occurs in the string

src/builtin.rs

@@ -303,6 +303,8 @@ impl Engine<'_> {
         self.register_fn("contains", |s: &mut String, ch: char| s.contains(ch));
         self.register_fn("contains", |s: &mut String, find: String| s.contains(&find));
         self.register_fn("clear", |s: &mut String| s.clear());
+        self.register_fn("append", |s: &mut String, ch: char| s.push(ch));
+        self.register_fn("append", |s: &mut String, add: String| s.push_str(&add));
         self.register_fn("truncate", |s: &mut String, len: i64| {
             if len >= 0 {
                 let chars: Vec<_> = s.chars().take(len as usize).collect();