improve error messages

- fix bug where `thread` function wouldn't error when being called on a non-function
- add compile error when a function in `try` is never used, because this is almost certainly a mistake and will cause mers to unexpectedly go down the "wrong" code path.
- some small changes so mers_lib can be used in another project

mers_lib/src/program/configs/mod.rs

@@ -84,11 +84,19 @@ impl Config {
         }
     }
 
-    pub fn add_var(mut self, name: String, val: Data) -> Self {
+    pub fn add_var(self, name: String, val: Data) -> Self {
         let t = val.get().as_type();
+        self.add_var_arc(name, Arc::new(RwLock::new(val)), t)
+    }
+    pub fn add_var_arc(
+        mut self,
+        name: String,
+        val: Arc<RwLock<Data>>,
+        val_type: crate::data::Type,
+    ) -> Self {
         self.info_parsed.scopes[0].init_var(name, (0, self.globals));
-        self.info_run.scopes[0].init_var(self.globals, Arc::new(RwLock::new(val)));
-        self.info_check.scopes[0].init_var(self.globals, t);
+        self.info_run.scopes[0].init_var(self.globals, val);
+        self.info_check.scopes[0].init_var(self.globals, val_type);
         self.globals += 1;
         self
     }