Fix some old Rust codes

Done by cargo fix using nightly-2019-10-04 toolchain. Fixed ... to ..=, trait object to dyn trait object

src/prolog/heap_print.rs

@@ -147,7 +147,7 @@ fn char_to_string(c: char) -> String {
         '\u{0c}' => "\\f".to_string(), // UTF-8 form feed
         '\u{08}' => "\\b".to_string(), // UTF-8 backspace
         '\u{07}' => "\\a".to_string(), // UTF-8 alert
-        '\x20'...'\x7e' => c.to_string(),
+        '\x20'..='\x7e' => c.to_string(),
         _ => format!("\\x{:x}\\", c as u32),
     }
 }