Add AttrsBuidler, change Attrs struct around some

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

src/obj/ctx.rs

@@ -1,12 +1,17 @@
-use crate::mem::{gc::Gc, BasicGc};
+use crate::{
+    mem::{gc::Gc, ptr::DynRef, BasicGc},
+    obj::Sym,
+};
+use std::collections::BTreeMap;
 
 pub type DefaultGc = BasicGc;
 
-pub struct ObjCtx<G=DefaultGc>
+pub struct ObjCtx<G = DefaultGc>
 where
     G: Gc,
 {
     gc: G,
+    globals: BTreeMap<Sym, DynRef>,
 }
 
 impl<G> ObjCtx<G>
@@ -14,13 +19,22 @@ where
     G: Gc,
 {
     pub fn new(gc: G) -> Self {
-        ObjCtx { gc }
+        ObjCtx { gc, globals: Default::default() }
     }
-    
+
     pub fn gc(&self) -> &G {
         &self.gc
     }
+
     pub fn gc_mut(&mut self) -> &mut G {
         &mut self.gc
     }
+
+    pub fn globals(&self) -> &BTreeMap<Sym, DynRef> {
+        &self.globals
+    }
+
+    pub fn globals_mut(&mut self) -> &mut BTreeMap<Sym, DynRef> {
+        &mut self.globals
+    }
 }