add mod and exp functions, tests, and also some snippets for common

patterns in the readme

src/stl.rs

@@ -283,6 +283,28 @@ pub fn static_stdlib(syms: &mut SymTable) -> Result<(), String> {
         },
     );
 
+    syms.insert(
+        "exp".to_string(),
+        Symbol {
+            name: String::from("exp"),
+            args: Args::Lazy(2),
+            conditional_branches: false,
+            docs: math::EXP_DOCSTRING.to_string(),
+            value: ValueType::Internal(Rc::new(math::exp_callback)),
+        }
+    );
+
+    syms.insert(
+        "mod".to_string(),
+        Symbol {
+            name: String::from("mod"),
+            args: Args::Lazy(2),
+            conditional_branches: false,
+            docs: math::MOD_DOCSTRING.to_string(),
+            value: ValueType::Internal(Rc::new(math::mod_callback)),
+        }
+    );
+
     Ok(())
 }