Manually implement Debug for MethodInst

MethodInst's self_binding was causing endless recursion issues, this
just skips over it and uses the normal formatting for it

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/function.rs

@@ -223,13 +223,23 @@ impl Obj for UserFunctionInst {
 // MethodInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug, Trace)]
+#[derive(Trace)]
 pub struct MethodInst {
     base: BaseObjInst,
     self_binding: ObjP,
     function: ObjP,
 }
 
+impl Debug for MethodInst {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("MethodInst")
+            .field("base", &self.base)
+            .field("self_binding", &format!("{}", self.self_binding.borrow()))
+            .field("function", &self.function)
+            .finish()
+    }
+}
+
 impl MethodInst {
     pub fn new(self_binding: ObjP, function: ObjP) -> Self {
         Self {