Add mem and obj modules, move to nightly

* mem and obj modules are roughly divided between doing memory-specific
  things and object-specific things
* Box::new_uninit() is a nightly feature and it's quite useful, so I'm
  enabling that for now
* Check out src/obj/attrs.rs for possible undefined behavior in the
  Attrs::new() function. I'm sure it'll be just fine.

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

src/obj/mod.rs

@@ -0,0 +1,44 @@
+pub mod attrs;
+pub mod ctx;
+pub mod dict;
+pub mod error;
+//pub mod native_fun;
+//pub mod num;
+pub mod str;
+pub mod sym {
+    pub type Sym = usize;
+}
+//pub mod ty;
+
+pub mod prelude {
+    pub use crate::{
+        obj::{
+            Attrs, Dict, DictRef, Str, StrRef, Sym, ObjCtx, Obj,
+        },
+        mem::{
+            ptr::{ObjRef, DynRef},
+            intern::Intern,
+            gc::Gc,
+        }
+    };
+}
+
+pub use self::{
+    attrs::Attrs,
+    dict::{Dict, DictRef},
+    ctx::ObjCtx,
+    str::{Str, StrRef},
+    sym::Sym,
+};
+use crate::mem::ptr::ObjRef;
+use std::any::Any;
+
+pub trait Obj {
+    fn attrs(&self) -> ObjRef<Attrs>;
+
+    fn is_marked(&self) -> bool;
+    fn mark(&self);
+    fn unmark(&self);
+
+    fn as_any(&self) -> &dyn Any;
+}