Add support for character constants

examples/simple_fn.rs

@@ -1,7 +1,7 @@
 extern crate rhai;
 use rhai::{Engine, FnRegister};
 
-fn add(x: i32, y: i32) -> i32 {
+fn add(x: i64, y: i64) -> i64 {
     x + y
 }
 
@@ -10,7 +10,7 @@ fn main() {
 
     engine.register_fn("add", add);
 
-    if let Ok(result) = engine.eval::<i32>("add(40, 2)") {
+    if let Ok(result) = engine.eval::<i64>("add(40, 2)") {
        println!("Answer: {}", result);  // prints 42
     }
 }